Solomon's Keep/YMMV

Demonic Spiders: Poison Mages. These guys fire out homing poison balls that poison your character and drain around 25 health (you start with 50). But that's only on the easiest difficulty- on harder modes, they drain even more! Although said poison cannot kill your character, it'll make you easy picking for the other Mooks.
- Hell, any mage enemy is this on Demigod difficulty. ALL mage enemies are able to cast Magic Shield, which allows them to take a lot of damage (even more if it's orange), making them one of the most durable enemies (without the shield, they do down very quickly). And then they rapid fire their deadly spells at you, some of which are unavoidable (lightning) or homing (ice, poison) or just plain damaging (fire)...
Game Breaker: A maxed out Flame Lash, along with a level 11 Battlemage skill (which makes your mana consumption zero), level 11 Siege Mage (multiplies your damage fourfold), level 9 Faster Caster (increases casting rate and mana consumption rate by 90%), and a +150% faster casting ring. You'll be doing over 12000 damage a second, destroying the shielded mages mentioned above and even bosses in less than a second.
Goddamned Bats: Ghosts. They fly around in circular motions very quickly and ram into your character. If they hit you, they slow you down for a couple of seconds. It also doesn't help that their wail is one of the Most Annoying Sounds in the game. Thankfully, they don't have much health.
- Imps can be this as well, they bounce around very quickly and come in large swarms. Their "bouncing around" sound is pretty irritating too.