This is a pretty simple battle. The Necromancer has 3 attacks he will switch between and they are:
First is a basic energy projectile from his staff. All you have to do is slash the projectiles as you run towards him and eventually hit him. He will either teleport to the other side of the room and repeat this attack or his will move on to a different attack.
Secondly, he will teleport on top of his skull platform and fire down a continuous green beam while moving left to right and he will not stop until you climb either side of the room and slash him or shoot him with your shuriken. Once you hit him, it will throw him to the ground and stun him briefly, this is your chance to get in a few hits.
Thirdly, he will teleport to the very top platform and cause acid-like rain to fall down from left to right leaving you helpless to attack him so you will need to take cover underneath his skull platform and wait it out.