I’ve been DMing now for about 6 months, for a now level 8 party of 5, with 2 monks, a barbarian, a paladin and a warlock, and recently I’ve been having a few difficulties. 

Firstly is dealing with overpowered players. Namely one of my Monk players is super overpowered, seemingly blocking my monsters from creating any real challenge for the party. He constantly knocks them down, restrains them, incapacitates them, basically using attacks which prevent them from being able to do anything, which can be really frustrating as a DM, trying to create a challenge for players. I can throw monsters and bosses at them way above their appropriate challenge rating and they’ll wipe the floor with them. Most of the time I’ll only get a few hits in per round then they kill it. I really don’t know how to solve this issue as everything the players are doing perfectly abides by the rules, without just creating a greater challenge, which also doesn’t work, as I will explain in my second point. 

Another big issue I’ve had is that the party tends to run away from more difficult fights rather than face them. For example, when introducing my BBEG I intended for an intense and epic battle where the players could learn some ino on the BBEG and progress the story. Instead they just ignored it and fled, forcing to me having to improve the entirety of the remaining session and lead to what I would consider the unfulfilling death of one of the BBEG’s henchmen. I don’t want to railroad my group but I feel like it’s the only way to actually give them a story worth telling. 

Generally im just feeling like the party is exploiting every barrier I put up, even when I think I’ve covered every base they find a loophole to get around the issues and make my life harder which I feel like is being a detriment to the campaign. I want there to be real stakes. I want them to feel like there’s an actual threat without them becoming too powerful and thinking they can overcome every obstacle by just killing it.  

Are you throwing multiple monsters at the party in an encounter?  A monk shouldn't be able to control an entire enemy squad and single monsters are notoriously easy to defeat compared to their challenge rating...without legendary and lair actions its one turn for the enemy vs. four to six for the party.

Never just have one monster fight the party at a time it's just asking for a bad time. 4th Ed had this thing called Minions they were a large wave of badguys that had 1hp. In my games, I buffed them up to half the max hp of the main monster/ threat and space them out all over the map.

Sometimes you have to railroad them sadly otherwise they will run away, now you can screw them over I did it and now my players don't just run.For fun and effect use something that can fly like an Adult Dragon

1) Let them run let them go anywhere they want.

2) Have the bad guy follow them 

3) have them chase them to the ends of the earth once they stop just at a town have the Dragon fly at its max height ( and out of range)  and unload on the town.

4) Reduce the town to ash and rubble then have the survivors yell at the party about how they can be so reckless to not kill it before leading to them. 

This will, in theory, stop them from running

In addition to the 'don't use one monster against a group', unless maybe it's one of those legendary monsters with the special abilities to mitigate some of the effects of being a solo monster.

Double-check the monk's character sheet and make sure he's doing everything right.

To the second point ... talk to the players and see what's up, and see why they're so inclined to run.  It kinda seems like they don't get that whole 'hero' thing they're supposed to be doing.

There's always consequences to actions.

Example. Player sees a person steal something and turns him in. Later on players see that man have his hands cut off in front of his family including kids. Horrible but that's consequences.

I gave my players level one characters to play while introducing a new player to the group. In a hard battle they were supposed to lose they ran. Three Theives ran back to their guild. What they don't know is that a few dc5 mobs are following them and will create some chaos later in the story.

Difficult terrain is a good way to give mobs a way to catch players running away since the mob can catch them. If mob catches a player they might turn around to help the player. Still running would mean a player being captured or killed.

Runes can help to make the magic users less powerful or step them of all their powers.

Custom mobs can help. If the monk is taking them down then the mob can have a reaction to that by engulfing the player taking them with the player taking damage and having to roll to get out of it.

Ranged enemies can surround players hearing the commotion of a fight. They can have bolts or arrows of entanglement or slow or paralyze.

Can have mobs charm a player.

Lots of ways to mess with players. If you feel the players are going to run have the mobs run to regroup on the players later surrounding them or leading them into an ambush.

They might prefer tactics to role playing and they may be better tacticians than you, which is frustrating.  Mirror the things they do to you.  Rival monasteries?  Study their weaknesses and exploit them.  Since they didn’t get info on the villain, he got info on them, and he’s going to use it.

It looks like they only have one blaster, think about a ranged ambush.

I should add, I’m inclined to applaud players who avoid risky fights, although they never get the big rewards without some risk.

Yeah, terrain to burn movement and ranged attacks should help against this party. Make them take a circuitous path and rain down on them from on high. Traps are handy too, particularly if they’re dashing to close ground.

As far as the running, I’d echo the above about consequences. You’re meant to stop the BBEG from killing the village but run instead? Ok, village is dead. Find what the players care about and go after that. Shrines to the Paladins god should get a strong response.

For the first, thing, vary up the types of enemies they're facing. Single-target incapacitation is good against single big monsters... what if they're facing half-a-dozen smaller ones instead? Do they have good AOE? Are they good vs casters, vs ranged enemies? Try different things to see what challenges them.

For the second, the players seem to be behaving reasonably. They fight something if they think they can kill it, but run away if they think it'll kill them! IMO, the solution here is to make sure the players have goals, and the BBEG is standing in the way of those goals. After all, if the only reason they're fighting the BBEG is well, because he's there and they're murderhobos so they stab things - yeah, they'll run away if they don't think they'll kill him.

But if they're fighting him because they want something he has - well, if they run away, they don't get the thing. Or if they're fighting him because they want to stop his plans - well, if they run, I guess the BBEG's plans are put into motion.

I think giving the players *motivation* to go into tough fights would work better than making reasons that they can't run away from them.

Ok, forgive me this but, right now you sound like you're fighting against the players, this is not the right way to go about it.

Your players run from what appears to be a difficult or unbeatable fight, YES!!!! That is smart, that is something most parties won't do, it's how so many parties TPK and complain about being railroaded. That is good survival instincts and should be applauded. However, there is always the chance that running has consequences and the baddies they ran from are now more prepared for the next, inevitable fight. The item the party needed is now missing or moved, another inconvenience is now added to the goal.

Your party has an answer to most of what you're throwing at them...so change things up. Most times I hear about a fight that is supposed to be deadly turning into a slaughter in favor of the players is because the DM doesn't do anything but trade d20 rolls. Do fights in waves, have smaller groups show up from different locations, have the enemies run and use guerrilla tactics, have them use the terrain to their advantage. There's more to a fight than swinging a sword, surrounding your enemy, and "Irish Boxing". I almost killed five level 5, fully rested, experienced players with 8 goblins simply because of height advantage, ranged attacks and Nimble Escape.

Use spells, use ranged, use varied types of monsters, use the terrain, and use your imagination. What is the reason for the fight? Why are these creatures fighting to the death? Do they really want to fight to the death? Is there a chance of the monsters getting reinforcements? Do the creatures know the terrain better than the characters? What advantages can you give the creatures that make sense for the situation? Make it more about the way the fight plays out rather than who rolls the biggest To Hit and Damage. If they run, they run. They lose out on valuable information, they lose out on loot and XP. 

You may have to improv the rest of the session, but you already know what you wanted the players to gain, so find a different way to give it to them. Do they need to learn of the BBEG's plan? Have them run across an abandoned research facility that the BBEG used. Do they need to learn of an impending doom? Have the BBEG's henchmen tell them, or some NPCs that escaped tell them. You don't have to scrap the session, just change how the players gain their information. Do they have to go back because of a McGuffin of Power that was in that location, have the local town Sage explain things to them. Having the world react to their choices is a much bigger way to influence them than you trying to create a situation for a specific purpose that the players can avoid.

Your monk cant get flying creatures as long as they remain in the air, out of his reach. So the bad guys start sending allies that can ride flying creatures, or which are flying creatures themselves.

For example:

An archer, using devils sight, and riding a wyvern, coupled with a ground based Warlock using Devils Sight, Darkness, Agonising Blast and Hex, coupled with a few lower CR minions (very low level devils) who can move in and attack with swords and spears, in order to take advantage of hex.

Now the party has to deal with magical darkness, while the archer rains down death from the sky, the Warlock hits them from the ground with agonizing blast and hexes them, and the minions move in for close range martial attacks.

For lower level battle, you could take away the Darkness or reduce the number of minoon, or you could increase the difficulty by giving the wyvern an attack as well, so it's no longer just a flying mount, or by increasing the CR rating of the minions.

Another way to increase the difficulty is make your party face enemies riding ground based mounts.  For example, facing a group of armoured knights using a lance or spear.

You could also have battles while both the party and the enemies on using vehicles or mounts, like maybe the party is riding on a cart and they are being attacked by archers on horseback.  Make it harder for the party to just stop the cart, by having the horses pulling it be spooked by all the commotion, so that they have to roll to see how well they can handle animals as well.

Other ideas are having the party fall into a trap or ambush, so that they are not prepared for battle right then and your NPCs get a free attack.

Bad guys are smart, and if they are being constantly defeated, they are going to start finding ways of neutralising the parties advantages.

The ideas already presented (wave attacks, have mobs use tactics and terrain, etc) are excellent.  Here's an article about kobolds that has some good ideas: here

Could also scale up your attacks.  If you've set up a "hard" encounter and the party destroys it, then throw a deadly one at them.

As far as the party avoiding tough fights..... I would have the BBEG's plot progress to the next level.  Adventurers are a BBEG's bane and when the adventurers don't get in the way and don't mess things up, then the BBEG just gets stronger (at least his forces get stronger, more entrenched, closer to goal completion).   For example:  Party was to get information on the main enemy encampment's numbers, composition, etc.  Unfortunately, they crossed into the territory at the wrong time and came under attack from two scouting parties.  PC's were forced to retreat without completing the mission.   Without that information, adequate defenses weren't put in place and a town was overrun.

Doesn't take that many instances of "running away" and not completing a mission for boundary lines to change, zones of control to shrink.  It feeds the process and raids start happening more often and are stronger.   Happens enough times and the locals will stop hiring the party.

I'll echo the great advice you've received above, with particular emphasis on 'fighting smarter, not harder'. You've singled out one of your players, and I'm not sure how he's being allowed to dominate the battlefield with such impunity? Have you checked that he's entitled to a lot of the abilities he's attempting? A quick glance over his character sheet might yield some interesting results. I'm not claiming foul play - but there're a lot of rules, and it might be worth double checking those around some of his more powerful abilities.

With that out of the way, have you thought about why your enemy mobs are being steamrolled? From the way you've described it, the party enters a room of appropriately levelled enemies, and they quickly get mown down, with very little time to fight back. I see this all the time, where enemy creatures are reduced to their stat blocks, and thrown at the party until the HP of one group reaches zero.

But consider thinking like a player - and picture your players' party as a group of enemies. How would you prepare? What traps would you lay? On hearing an approaching group, would you try and hide, find an advantageous position, or retreat to find back up? Your npcs should do the same.

The player's party composition is very melee focussed - so allowing them to get in the face of your creatures will end badly. Other have suggested considering creature type, such as flying creatures - which is fine, and can lead to a satisfying conundrum for your players - but relying on immunity can feel very railroady, in a similar vein to simply making a creature immune to damage. It might be worth considering how to use all of the creatures in the MM more effectively, and then throw in a few frustrating flyers to keep them on their toes.

In regards to the party fleeing obviously dangerous situations... that's actually smart. Why fight a battle they think they'll lose? Especially if the stakes aren't established. That's not cowardly, that's just tactics. So think of situations that either don't appear particularly dangerous from the outset.

For example, a group of goblins sit huddled around a campfire on a dark night, clearly guarding a chest of loot from a raid on a trade caravan. An easy mark for the unnoticed party, surely? ... Well the group of goblins are actually bait. They're prisoners of a nearby Orc tribe, to lure would-be-adventurers... and if they'd spent a bit of time assessing the situation, they'd have noticed the shackles around their ankles. If they rush in to sneak-attack the goblins, they'll find themselves up to their necks in Orc arrows, traps, and berserkers. If they're smart and assess the situation, they'll get the satisfaction of outsmarting the orcs... and could lay a trap of their own.

Finally, it's important to have obvious ramifications for avoiding a fight. I've found it very important to establish the stakes fairly obviously to the party. If they flee what they think is an unwinnable fight, and then find out their village was razed behind the scenes as a result, that's not a satisfying conclusion. Tension is lost if it all happens off-screen.

First, when facing the BBEG the players should have a go plan. The Dm always presents options to how to go about it. Like a weakness, maybe the BBEG is weak any weapon made by a certain person or metal. Then there is environment. Not everyone wants to take on someone head on. Maybe make it so they can set up the fight to their advantage. First DM mistake is setting up the final scene before the players get their. You should set up for the players to win not to be on a stage. Lets say your BBEG is a wizard who rarely leaves his tower unless some dude makes a "yo mamma" joke. His weakness is kermits banjo being played, as it reminds him of much simpler times.

The players can now draw the BBEG out with a "yo mamma" joke and trap him in a area of their own choosing. Maybe they set up mine fields and nets which will hinder the BBEG. It gives the players a sense of advantage. The next option is the weakness, if the players pay kermit's banjo then all the evil in the BBEG melts away. Thus giving the players the choice to find another , easier way of taking the BBEG down. The weakness of a BBEG is usually protected, or lost so it will force the players to work to find it. That also means you should make it clear that their is a weakness. Never present a weakness once in a cryptic or secretive way and the at like you out smarted the players when you failed to clearly state their was a weakness. It's only when the players choose to just head into the villains lair head on without a plan can you really plan for a big fight scene. the villain is in their element, meaning they have traps, spies, monsters. Our wizard will have magic explosions go off every time the players say "yo mamma" and a displacer beast will stalk the players through the lair. Also the most important element should be that the lair is set to trap the players inside once they enter, and may only open upon the BBEG defeat. Of course this is a dick move of a DM but then again you can always make the BBEG chase them. 

Now about the over powered players, implement a role that will force the players to do something or suffer some kind of condition making them loses levels. Like the players could be forces to train during downtime as to keep themselves in shape. Failure to do so will result in a loss of levels. Also don't use exp and instead use milestones. If a player is to over powered, you legally do something to balance the odds. Just be subtle about it and don't be a dick about it or else you will notice a decline in players. 

Quote from Fakinguy >>

You should set up for the players to win not to be on a stage.

Fakinguy has some good points, but I'll disagree on this.  Encounters should be winnable, OK, but I fail to see the point in going through all the effort to run a campaign if you design it so every encounter is an "easy button."  Admittedly, I'm more "old school" and believe in challenging the party.  A mini-boss encounter a few weeks ago had both clerics go down and both rogues had to break stealth to rescue them.  The group still talks about that.

As far as being on stage..... sometimes they are.  Sometimes the BBEG is putting them on stage, like a puppet master.  Acts of "good" can be used for "evil" as well....