Find a way to give him astronomical (as in, something the player won't be able to even get near with their current resources) move speed and make terrain with lots of obstructions, walls, pillars, etc. Also, give him Shot on The Run or Move-By-Attack or something of that nature.
If you prefer an airborne encounter, the same can be done with spells and, for instance, fountains of flame, falling rubble, flying artworks and furniture which are quite at home in high fantasy, towers and walkways... whatever you like.
Now, his every round consists of moving past the players, chopping at one of them (he won't provoke AoO from the one he's attacking, but he will from anyone else he's moving by) and vanish in the nearby obstructions again. Provided he moves fast enough and the terrain is good, the only moment at which the players will have a line of sight or effect to him will be as he shoots past them.
Perhaps supplement with a magical trap or other systematically re-firing effect of Incite so that they can't ready actions, either.