I would rank the levels
1 > 4 > 3 > 2
1 is the shortest, and only has gay robots and trolls, boss is cake
4 is pretty lengthy, but it just has zombies which are pretty easy to dispose of, just use far away Fierce Attacks. the wraiths are annoying, and suck your life if you get too close, and explode on death (roll to avoid). the floating fat fucks are even worse, resurrecting dead enemies around you that are in proximity, but overall it's not bad. expect to grind here a lot if you have a quest related to the undead, since theres no other level that has them. i had to kill 4000 undead for my Level 3 charm achievement.
3 is ok but it's all annoying Dark Elves (the green ones that shoot at you), and Thor is useless but he distracts enemies. the boss is just a giant troll that you cant mount, and you have annoying smaller goblins after you (good way to get health if they drop it)
2 is just annoying and seems like it goes on forever, and has Dark Elves plus the annoying giant spider trolls which cannot be mounted. the boss is the worst in the game. because of that, i place it as the worst level.