I prefer running this dungeon with 5 characters, none at level 50 or 65, and I'm doing all the levels in order to get the 3 Growth eggs at the end (5 Growth Eggs = 1 Greater Growth Egg, which is very nice). Quistis and Selphie will probably reach level 45 or so when the dungeon ends, and I'll just have to give them a few eggs to push them to level 50.
I'm currently leveling up Squall (62), Paladin Cecil (59), Sephiroth (58), Quitis (43) and Selphie (41).
I forgot Irvine you say? Ahahah. No. This guy is utterly useless unless I somehow get a good 5* gun (and even then, I'm not going to do much with a single synergy). He will stay at level 29 until I am forced to use him.
Relic-wise, I need ranged weapons badly (not to mention 5* Light armor and Armor; I have enough shields DeNa, thank you...).