In order to fight the lv 50 version of Ifrit you have to get done the quest (level 25 fight) It Kills Whit Fire obtainable from your GC.
after this you will have a new quest from Louisoix in Apkallu Falls Ifrit Bleeds We Can Kill It after this, you need 5 Inferno Tapers for each fight WON, obtainable from Amalijas in Zahar'ak Stronghold