New DM to 5e and I'm looking at my campaign's characters and one is playing a character with Expeditious Retreat which enables Dash as a Bonus action for 10 minutes after cast, which enables another full move on that Bonus Action.

Barring me managing to break their concentration, it seems like this presents an opportunity to the character to Move from full cover, Attack/Cast/Ranged Attack, Dash back and suffer no real risk.

Am I missing something?  Or do any creative DMs out there have suggestions to avoid this being spammed and making things less fun?

When your players meet new opponents they can indeed run over enemies. Once the enemies get a better understanding, gather more intel, on the group... Then prepare traps that reduce movement speed to close to 0. Have enemies use trigger attacks. Line regular opponents with a few using reach to try and utilize Opportunity Attacks and Reactions of the enemies against the player. Have enemy spellcasters  turn the battleground into difficult terrain halving the players movement speed. Use obstructions like Force walls and such to create blockades and isolate that speedy character. Lots of things you can do.

So Rogues can do this at 2nd level (without the need for spells). Both of these situations involve a weak character that does high damage trying to maintain safety. The easiest way to get around this (or at least get rid of the "I'm safe") is have enemy group 1 attack from the front (fighter/paladin/barbarian take the hits and keep their back line safe) then after a round or 2 have a 2nd enemy group (patrol size maybe, returning to report) and attack from the back. This then gives the Player the option to risk hits from in front OR hits from behind.

Another option is to have the ranged enemies hold their action to attack the Player when they emerge to cast their spell. This could result in them taking 2-5 bow/crossbow shots before they have the opportunity to cast their spell (which could down a wizard, or at least make them scared to run out)

Yet another way is change the terrain to benefit the monsters, e.g. an ambush in a slight valley where the archers can shoot down on everyone or a tight corridor meaning the wizard has to be close to the front to do anything (and therefore in risk).

4th way, make the terrain actively hostile, e.g. there are several geysers around and they could blow any second in a 5ft/10ft radius and have that near the full cover areas. It will certainly be a shock if they didn't look around carefully as they entered.

5th way, have a gate block them off as the monsters signal an ambush, cutting off the main parties retreat and leaving the wizard to fight 1 or 2 lesser creatures, and thus giving them the fear of staying too far away from the rest of the party.

Final way I can think of at the moment, give them less time to rest between encounters (a large hunting party is at their back, they need to get to place X in 4 hours to save the village, etc.) This will mean that using up the spell slots for that spell will give them a disadvantage in the upcoming boss battle.

Just remember to make sure you're not making the spell/ability useless within your encounters. They should both be rewarded for using it well (which is that they don't lose hp) and penalised for straying from the group (danger from flank attacks, or being cut off by a terrain feature like a lava flow). Some areas will make it easy to 'abuse' this ability, but that doesn't necessarily mean that it's bad.

One thing to consider is what the players think of the strategy, if the other players are annoyed that the caster never gets hit than you probably need to deal with it, but if all the players like the strategy than there's not much of a problem.  In my campaign I play a Wizard and I've gotten knocked unconscious several times, so the members of my party would be more than happy with me using this strategy.

But if you want to counter the strategy there are several options, the easiest being just not give them the option of having full cover, if you're in a cavern or long hallway there's nothing to hide behind, (except for other party members.)  You could also have the enemies try to get past the front line and go for the caster.

You've already gotten a lot of great feedback on this, so there really is not much more to add. The biggest thing I wanted to say is to not worry about it too much. In your example, expeditious retreat requires concentration. There are sooooo many much better spells that require concentration, so unless the player really just likes this one same battle plan, chances are they will start using different spells to consume their concentration. Also, with the 5e structure of "spell slots", they could only do this so many times before they can't anymore, given that you have multiple combats a day or what not. 

It has been mentioned already, but I'll just piggy back on it. Rogue's can do this pretty much indefinitely, along with many other bonus actions come second level with their "cunning action". As a DM with a rogue in the party, I can speak from experience that this tactic is really not exploited that much. 

Again, as mentioned, there are a ton of work-a-rounds. Don't provide cover, make that player running away put the rest of the party in danger, have an enemy fixate on that player: all these make the tactic null and void. 

So, just to kind of reiterate once more, don't worry about this, or even other "spam worthy" tactics. You'll find that in D&D that it is more uncommon to spam certain tactics, as that's honestly pretty boring in a format like this where there are so many other awesome things to do. D&D is also not a game like Street Fighter where you can fight the same enemy and spam "Hadouken" and win every time, that is as long as you, as the DM, don't continuously throw the same enemy, with the same tactics, in the same terrain at the players.

This tactic is also one of your least worries...wait until one of your players learns "Fly" and says, "don't worry guys, I will go fight those enemies alone and just rain firebolts down on them from the sky, out of range until they're all dead." 

Thanks, Folks!  I appreciate the feedback and excellent ideas.  Yeah, I have a bit of time before I need to worry about it getting too out of hand, but I do have a Rogue and an Aarakocra Ranger as well, so I suspect they'll force me to get creative.  I appreciate the head start!

If your players are using good tactics, good for them. Just make sure you play the bad guys to their full potential. Goblins are a lot more challenging if you use their Nimble Escape ability and terrain to good effect.

Quote from Giblix >>

Once the enemies get a better understanding, gather more intel, on the group... 

 

 This right here is so important. I think we as DMs are sometimes afraid to use tactics because players many times (wrongly) assume that enemies are fodder thanks to (especially early) video games that made enemies, particularly grunts, pretty dumb. 

Anyone that's seen a couple of battles or been on some hunts, including goblins and kobolds, will use some strategy. Keeping it specific to your example, and leaning on a previous poster, if the PC wizard exposes himself, attacks then takes cover, at least a couple of frustrated baddies will hold their attack until he pops his head out again.

The trick is slipping in this nonaction without bringing attention to it, because, whether we like it or not, people meta game and although said wizard has no idea that the baddies held an action, so (s)he in theory should repeat the tactic, the player her/himself may have picked up on it and direct the PC to do something else.