The best thing to do is to have Squall at a very low level (as all monster are based on his level) and your GFs should of had most of their abilities learnt by now. This making you powerful at any level you are but the monsters are weak as Squalls' level sugests.

EDIT: The only GF you cant get from Ultimecia Castle is Diablos, if you dont get the lamp at the start, then he's gone for good, unless you got a pocket station.