Sorry for the late response, I did not notice until now.
I don't see anything wrong with the script, but I'll have a playthrough just to be sure.
Basically, kill as many Heroes as quickly as possible using the Vampire that hits very hard. Beside the Avatar himself, only the high level Heroes that spawn with him are a threat.
Also, there is a a limit on how many Heroes there can spawn. Your allies combined can boot up 3x the numbers of minions, so in theory its possible to simply outnumber the Heroes if enough time is passed.
Looking back at the script, maybe the interval for the respawning Heroes can be a little bit longer. But let me know on how you are doing.