I know your problem well ^^ cause i am a Summoner too . But this is the only rly hard stage for a summoner. And it is hard but not impossible. Just recast fast u summons when they do it and all fine. I think it has to do with some dot they get perhaps poison. I will replay the stage and will look into it.
"Alle Lebewesen außer den Menschen wissen, dass der Hauptzweck des Lebens darin besteht, es zu genießen."
"All animals, except man, know that the principal business of life is to enjoy it."