Is there actually a proper way to fight mimics? They always seem insanely strong and move so fast too
Mimics aren't really that difficult at higher levels. However, you want to have the following:
As high an Arcane rating as you can. This means your spells will do more damage. Don't worry about cooldown. It would only be an issue if you only had 2 or 3 spells available. You should have multiple spells before attempting to fight a mimic. Mimics pretty much take the same amount of damage, regardless of spell type, although I've noticed that Fire seems to do a little bit more than the others.
Have several Tier 2 heal potions. Use one whenever your HP drops below 50%. I usually only have to use 1 or 2 before I kill the mimic, but you may need 3 or 4 depending on how your battle actually goes.
Have your HP as high as possible. Have the Max HP boost skill on from the silver or gold Health Necklace (preferably the gold one if you have it since it gives a 15% boost). If your health is over 4K already, you probably won't need the boost.
Basically, just keep hammering it with your spells until it dies, using your heal potions whenever your HP dips below 50%. There is no "quick" way to kill them, but they'll die eventually.