You can block most of the Necromancer's attacks. Or go an easy level, find a magic refiller thingy (forget what it's called), beat the level, and go back and try to beat the Necromancer again. --- "Sticking feathers up your butt does not make you a chicken" - Tyler Durden (Fight Club)
Yeah, necromancer isn’t so tough…just get through it. Dodge to the side when he blasts forward, and jump on his ground-pound attack. Get in a few licks and be ready to parry. Use the area attack combo when he summons zombies. He’ll go down quick.
Also, if this is your first playthrough, there’s some shadow and light gems out there to collect and those refill your magic.