Yeah, but that's like screwing the whole party over. And I don't give XP per encounter. I stack them up per session, if they add up, I just reward them with a level. The whole point was making the fight challenging. So much for that ...
Well, there's always the option to ask him to stop doing it.
or you could just do the old "what the players can do so can the NPC's" for some reason none of my players chose save or die and for that reason only BBEG's who realy must to fit there concept does so (Necro or evil cultist of Death)