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For those of you who are having difficulty, I have the help you might need. The fight I just had against him ended without a single undead being summoned.

The first thing you want to have is Fire Magic up high enough so that Fireburst can ignite the enemy and continue to burn after the spell is done. Now, Xarant doesn't summon undead while the party is in any of the 8 tiles surrounding the tile he is standing in. As soon as you you step out of those, he drops his swords and summons undead. He will summon them whenever he sees you at least two tiles away from him. So, before stepping on the plate which opens the inner door while closing the outer, cast invisibility, and go in, take a step to one side, and advance so that you are one step in front of and one step to the side of Xarant. Now, all you have to do is take a side-step to the side of him he is not facing, blast him with a Fireburst, and step back to your previous tile as quickly as possible. He will likely get a hit in even from the side. With any luck he will catch on fire and take damage while you stand still. He may turn from facing the gate to facing the side you're on from him, but as long as you stay in the 8 tiles around him, he won't take any steps or summon any undead.

Good luck. I hope this helps you.