I had the exact same issue. Had no real problems with any other fights including Ifrit but this one messed me up. Leave it a couple of levels, go all out attack and make sure your gear is up to date. Don't run ahead, kill the mages asap and pray you don't get the lancers attention. Went in capped to 24 and it was simple. Had been pushing for Free Company access so hadn't unlocked any cross class abilities. I think they might help