Tough it out. If anything, every time one of them revives, they revive with less max HP, making it easier to kill them simultaneously.
Grah's grand quest is not hard at all; just time and energy consuming. You have to run the quest four times (Grah's party has to fight each of the four raid bosses at least once, among some other things). The "quick and dirty" Grah guide on reddit is perfect to follow to minimize time spent on it. If cost/units are an issue. 7* Selena and friend 7* Selena can easily manage and kill the fire raid boss. That will help free up units and cost to delegate to other parties.
XI Wiki


