I ask this because I can see it becoming a ploblem. Recently my players steamrolled an encounter by banishing the enemy mage on turn 2, the cleric being a beast in concentration and AC made it so the other enemies could make him lose concentration, and when all enemies where dead they waited for the mage to reappear to destroy him in one round. 

Now I was thinking of an encounter with a fiend, but I feel like the players might trivialise it with the spell. How do I deal with a team of six where one guys always has the spell prepared, and two might gain access to it in the future if they choose to? What are your thoughts on this? 

Counterspell

Edited to add: it's not an irreconciliable problem. things that you can do that don't involve this are to roll high on initiative/surprise the party and knock out the spell casters first. What are your bad guys doing, monologuing? Heck, cast banishment on them!

Creatures with legendary resistance, who can auto-pass the saving throw, also would resist it well.

Single very powerful monsters - if they're fighting a single dragon, banishing it just annoys it since it'll just return and eat them in a minute.

Groups of roughly equally-powered enemies - if they're fighting eight monsters, using a concentration spell to keep ONE of them out of the fight isn't that big of an advantage.

I think your party is using banishment exactly as it's intended - a mid-level save-or-die spell for particular situations. Taking out high-priority targets is exactly what they should do with it. This sort of use is most effective against an unbalanced group of enemies, like when there's one boss and a half dozen minions.

Note that this use of banishment really just a different version of the many spells around that can take a single monster out of a fight. Hold person/hold monster, power word stun, tasha's hideous laughter, heck LEVITATE can do this against a melee enemy, etc. Banishment is better in that the target only gets one save, but worse in that the target can't be damaged for the duration. I don't think you need to find a way to fight banishment specifically.

Legendary Resistance

Counter Spell

Dimensional Shackles, which may work. The text says the shackles stop a creature from using any form of teleportation or plane shifting magic, but they can step through a teleportation circle.

If the fiend is reasonably high level and the fight is taking place in a place that it has prepared, you could make it so that the Fiend had a means of traveling back to the material plane hidden there. The players may feel that is a cop out unless you make it part of the fight that they have a limited time to fine the summoning circle, get through any defenses, and destroy it or else the fiend is back. 

Creatures that are native to that plane - even if they shouldn't be.

In addition to what has already been said, many fiends either have a high charisma score, are "proficient" in charisma saves or both, so are less likely to be banished in the first place.

Multiple enemies. You need to keep it banished for the whole one minute, and every time you get hit you have to save or the thing you banished comes back. Plus, as Acme pointed out, the things you really want to banish tend to have high Charisma, proficiency in the save, and magic resistance. 

This is the question for the ages for 5e.  Banishment is available at such a low level, that unless the DM specifically tries to plan around it, PC's can use it to steamroll most encounters.  It's so cheesy, as a DM, to have to ensure that there is a counterthreat to Banishment in every encounter.  Seriously, fighting a T-Rex?  Banish, prepare, ambush.  Big baddie fighter with minions?  Banish, slay minions, ambush.  I really hope that we get a fix with this spell.  Oh, and hypnotic pattern.  That thing is incredibly tipped in the party's favour.

Basically anything that should be banished can't be banished lol.

Throw in an enemy that does the same thing!

I think a lot of the times, DM's forget that if a player has a really powerful ability, that THEY can use it too.


Either that, or just let 'em have it! The banish-er can only do it as many times as they have spell slots and If the enemy saves against it the spell is wasted. When the combo of banish and readyied actions goes off, its amazing and it feels great to the players. but it has a significant chance of just NOTHING happening and the spellcasters turn is wasted. I'd say just let them have it. It might be a big threat to monsters now, but as they level up and creatures start getting legendary resistances, or they start fighting creatures with high saves, or fighting huge groups, it won't work. so just let them have the beatdowns for now lol

One funny trick I pulled for a boss battle was having the battle shift between one plane and bosses native plane.Beyond some shifting rules of reality it also meant that if the boss was banished they would basically get blink for free.

Going into a spell-scarred area? The banishment is trickier than normal - mage needs to make concentration checks every round against an increasing DC or the spell fails early.

Going into the lair of some wizard? The place has been protected protected by Private Sanctum blocking all teleportation and planar transport -- and yes, this includes that from Banishment.

A mysterious big bad beast of unknown origin? A monstrosity that is bound to the current plane, unable to leave it by any means. Or the creature is so naturally capable of shifting between planes that they cannot be teleported/banished/plane-shifted/etc against their will. I have a very vaguest inkling there may be an official monster with some sort of effect that is similar (but don't quote me on that, memory super extra fuzzlefluff on that one). 

The enemy has set the lair to release spells like Synaptic Static or Sleet Storm that make concentration difficult.

The enemy has Contingency set to cast a 5th level Dispel Magic on themself if ever under the effect of a spell they did not cast upon themself. 

The party mage banishes the enemy they see - and the spell fails, as they're targeting an illusion. The real enemy pops up from behind, targeting that mage.

Then, of course, the usuals: Counterspell , Antimagic Field , Globe of Invulnerability etc.

Banishment is a great spell for gaining control in combat. It is not infallible and there are many ways to deal with it. And as others have pointed out - this isn't a problem specific to banishment as there are many control spells that have similar effects, sometimes stronger. Magic is strong. You, however, are the One-Above-All, and all of that magic bends to your whim. you have an infinite number of ways to handle all sorts of magic problems. There is no spell in D&D that a DM cannot figure out how to factor into encounters.