Fayt and Luther is the hardest pair in the game, IMO, since Lenneth and Freya don't team up for some serious pwnage (at our expense!).
They become much easier if you wait until the point before you fight Lenneth at floor 210 of Sphere, where many level to 255.
Earlier encounters with those two usually need a lot more strategy. In my last file, I pretty much had to wait for Evil Fayt to use a minor attack so I could Stun AAA him in order to safely damage Luther. Took forever until one of them died <.<
Heard Air Raid does really good knocking both of them down after dodging a few attacks and sneaking up behind 'em
