How to Unlock Necromancer
Use the guy with the dogs I did so and it went smoothly.
Go for explosive shots. Level him up until you got his 2nd weapon.
Use all kind of shots and explosives he has (no mines, bombs).
Get all possible debuffs poison bleeding etc on you spells bec you have tons of stuff that deals high initial dmg and spreads around get a bigge rarea of effect and additional spellcasts.