Trolls!

[Plane Sailing]

Another option is to take advantage of the advice in the MM to make regenerating creatures more effective when faced by fire and acid - turn them into 'hit and run' creatures.

Because of their organisational structure, they don't flee fire and acid - but they will attack, then scatter, regroup and attack again once their regeneration has topped them up once more.

Also notice that burning a troll which has been reduced to 0 by sword blows doesn't prevent it getting up. It is ONLY if it is reduced to 0 by an attack which is fire or acid does it stay dead.

Taking advantage of these two things should allow a party of trolls to still cause problems for a PC party even when the party has lots of fire and acid available.

 

[Beckett]

The wizard in the group I ran Trollhaunt for was big on fire spells; scorching burst, fire shroud, fire burst (? it was a Burst 2 encounter). The fighter ended up with the acid/poison sword. The cleric had an old flaming maul to pull out. Only the strikers didn't have a ready source of flame.

I still got to use the regen on the trolls. Some fights, not much- fire shroud catches them all, I can't make my saves against the ongoing damage. Other fights, I got lots of regen; when three trolls are spread out, a scorching burst can't hit them all. Sometimes attacks miss. Sometimes the barbarian would chop one down, and it would stand back up before anyone could apply fire.

Were the fights too easy because the party could often stop regeneration? Nope, trolls still have a mass of hit points to work through. What made the fights easy was the trolls are all brutes, and I was having a hell of a time rolling high enough on their attacks.

 

[Plane Sailing]

Sometimes the barbarian would chop one down, and it would stand back up before anyone could apply fire.

Notice my second point? Technically, the way a trolls regeneration from the dead works is that unless it was taken to 0 by acid or fire, it WILL get up again.

If the barbarian cuts it down, you can't just apply fire to make sure it stays dead - it wasn't reduced to 0 by fire, so it gets up again.

Makes them a bit tougher!

 

[Beckett]

Notice my second point? Technically, the way a trolls regeneration from the dead works is that unless it was taken to 0 by acid or fire, it WILL get up again.

If the barbarian cuts it down, you can't just apply fire to make sure it stays dead - it wasn't reduced to 0 by fire, so it gets up again.

Makes them a bit tougher!

Technically yes. I opted for the interpretation that fire could be applied after- still needed a hit, would still take an action. I felt if I required the hit that took the troll down to 0 be acid/fire, I'd end up with part of the party twiddling their thumbs, waiting for a fire user to step up at the end of the fight. It would make them tougher, but I also thought it would make my game less fun.