What type of creature will this ability be used a lot on? I'd prefer it being a slightly dangerous enemy also, so that the ability isn't wasted.

Goblins. If you'll be level-squatting the later enemies won't be dangerous at all.
If not, it's still the best choice.

Yuan-Tis are a real menace in the later episodes, and getting help against them is great IMHO. Does it work against all kind of yuan-tis? (Half-breeds, Pure bloods and Abominations, and maybe even Histaachi?)

Half-Dragons can be really annoying as well. Goblins are always a good choice as you kill sooo many. I hope it helps against Half-Goblin hordlings as well. (which hurt a lot!)

It's too bad I can't choose humans like in the pen and paper version. :(