Grimrock

large fights involving different kinds of enemies will make for challenging game play (+ enviro hazards as Azel has said)
Around 3 or 4 different monsters - each with different behaviors and attacks is usually plenty challenging
We have so much variety with beasties now too

Emera,
Skel Commnader is a good monster to test custom brain work with!
Working with the brains is quite fun - check out:
1.the asset pack monster definitions
2.script entities within the main game ( u open LoG2 up in the editor )
3.brouse through the functions for Monster and Brain components

Im anticipating some stella custom works from the community with custom monsters,
keep your eyes peeled + good luck!