It is still purify fight, its uldyssian that was changed to normal damage.
Fortress is to apply slows target, so they move slower = easier to control them. But you don't need to really do it, there is nothing else to do during that fight.
Mendeln can die to rathma attacks very quickly, if you just let them fight and not do anything fight ends in like 20s, where Mendeln eats multiple shots from rathma and dies. Sometimes their attacks can get desynched and you can't even see them (black wave thing). Each of those takes decent chunk of hp.
It is Rathma that can't die fast in this fight and you must grind him down slowly, while keeping Mendeln alive, just to properly time that final rathma shot to kill him while hes at that 1% hp so after that he dies to your purify within 60s so you can cube your charm.