He consumes magic. That's about it. 

Magic that's bound to things- like to a gemstone or something else of value, that can hold a lot of magic on its own- will take longer to take away. Think of it like an anchor and rope- he takes all the rope in the world as he gets close, but the rope that's tied down to something is much harder. Necromancy involves reanimating the dead, meaning that you're going to be putting magic into a corpse to serve your bidding- which means the magic is tied to it. In mortal form, the ties may be strong enough that he can't suck the power out of it while fighting. Arrows are easier, because steel broadheads aren't really enough to 'tie' the magical properties down.