Ol' Willy
Arcane
It's easy regardless; the combat encounter design is just not grognard friendly.The same people telling you to multi-class and tell you about the best combos will also tell you the game is piss-easy.
Humanoid enemies are complete pushovers, be it Draj guards, ssurians or bandits. The only worthy human enemies are Draj elite troops and you will encounter them only in the very end.
Magera and half-giants are dangerous in close-quarters, but could be dealt with and latter are very rare.
Shadows fucked up my half-giant party real good - until I googled that AD&D treats shadows as undead and thus Turn Undead spell works on them... and three of my half-giants were Clerics.
Babau are weak but annoying with their "destroy items on the first hit" ability. Vrocks are dangerous, though.
Defilers are not bad, quite dangerous, but they're squishy, don't use crowd control spells and don't cover themselves with meat-shields, therefore easily dispatched.
Spider-mages are cool, Dagolar slimes are super annoying, but you fight them only once.
Wildlife though... entirely different level of difficulty. Bulletes, Stalkers, Xorns, Mastyrials, Wyverns, Rampagers - all can fuck you up very fast. But you can just run away from them, for they spawn randomly.