helpo1: In fact, there is not a "major" enemy in the game. The final boss is a bit harder to defeat, but you'll be terribly OP'd by then (which is quite a standard in Gothics/Risens). For me, the hardest fight in the whole game was the one with the Crystal Titan, but only because of the tremendous lags my old laptop experienced.
Out of the "regular" enemies, I actually think scavengers are amongst the most dangerous and annoying ones, mainly because of their attack "patterns". I hated them more than Shadow Lords, Leviathans and golems.

Also, if you ever have problems with combat, remember these three things:
- Roll a lot;
- Companions are immortal;
- R3 uses a two-hit system*.

* (Enemies always start targeting the last person who hit them twice in a row. So if your enemy is targeting a companion, you can use the hit-wait-hit-wait... strategy until your companion gets knocked out (which usually takes a really long while). You can take out a Shadow Lord with no combat skills and a lousy weapon this way.)

For the record, I managed to get to the final boss with these stats: Melee (15), Ranged (15), Cunning (105), Influence (60), Toughness (20), Dexterity (105), Magic (15), Spirit (15) and (IIRC) no combat skills learned. Then I used my 215250 Glory points and 105'd everything - and still, I had no problems with combat during the whole game.