As for Roy taking down Xykon, he's using the wrong weapon. He should have got that big wooden club enchanted instead.
Consider the damage he's doing to the dragon with each blow.
2d6 (two-handed sword) + 5 (+5 enchancement bonus) + 2d6 (undead bane) + 5 (est. 16 str) + 12 (est. power attack for 6) + 2 (Roy is known to have weapon specialization) = avg. 38 damage.
Roy is presumably about 13th level. Unlike a zombie dragon, he probably can't power attack and still consistantly hit Xykon, but just unloading a normal full attack is like 78 damage.
And assuming Xykon is roughly a 17HD lich, he's only got 111 or so h.p.
Except, Xykon has DR 15/bludgeoning and magic. So instead of doing about 26 with each blow, Roy only does about 11 to Xykon. Roy needs at least 4 full rounds to take down Xykon by himself. And Roy can't be expected to make his saving throw every round against save or die effects, even ones that have Fort saves. Besides which, Roy even with the save took like 29 damage from that. Roy is just waaaaaaayyyyy over matched going one on one vs. a CR 19 creature, even without a zombie dragon thrown into the mix.