I think two tactics would work:
1) Corral the archers in a corner and defend. A Castle and a Rampart would probably be suitable here. It's necessary to have the Tactics skill, and ideally, the Necromancer shouldn't have teleportation.
2) Kill the skeletons in the first turn. Only Crag Hack can help with his mega-damage. I tried it myself – it worked once. You also need a lot of Haste and the Tactics skill. And the final battle shouldn't be sooner than a month.