It's easier in single player, because the crappy co-op mode in this game doesn't even give you a full view of the arena.

I recommend you buy any master melee weapon (preferably something faster like a katana because it is great for crowd control) with 4 augment slots and get its damage as high as you can.

You're likely to slaughter everything pretty quick, but I'd recommend using a master flintlock to attack the necromancer because those headshots hurt.

Keep using the time control spell at level 5 to give yourself more time. thats the easiest method