Just finished this fight (annoying long) but tbh the void dragon never attacked Astarte, only the minions attacked her and the void dragon only attack my Knight (with full resistance...only normal hit dmg for 250+-)
Astarte just stood there spamming Bless+Minor Heal like 25man raid ungeared priest -_-
you should try to freeze the dragon...freezing is OP in this game, especially because here is no diminishing return.