When i tryed fighting ahriman i lost about 7 or 8 times once i beat him went out of the area only to hit a sten needle trap and have everyone die. But thats a different story anyway at that point in the game i had given my vaan 3 quickenings so i summoned maetus 3 times in a row and let him do the work, just make sure maetus doesnt die tooo quickly. Anyways when i did this i got him to about 30 percent hp and then use quickening to knock the crap out of him. Makes you feel reeeeal good when you beat him even better than when you beat penance.