erm....
Get the Lion Heart gunblade, use the Lion Heart limit break.
Game Over.
edit: Ok, so apart from the fact that the game is rendered far too easy by that one move, the Junction system is actually incredibly simple. It's just explained by a ridiculously overlong tutorial that cannot fail to make you go: eh? Especially considering that it drops it all on you at a point in the game when you don't have any bloody magic.
It's simply a case of 'drawing' high level magic (same as most traditional FFs) and using them in the junction slots for your important stats (Str, HP etc. though broadly, healing spells go in the defence slots and damage spells go in attack stats), but the Auto setting will probably do that for you anyway. You get those junction slots by carefully managing your GF's growth and assigning GFs to characters so to avoid overlaps (so everyone ends up with as full set of Str/HP/Agi-J as possible).
You draw much of the highest level magics by finding the 'Islands closest to Heaven/Hell' and running around pressing the X button like a tard. Though it helps if you have the encounter-nil (it's not called that, it does that) GF ability learned, because all the most horrible enemies are there... though once you're strong enough, they often have much of the nasty magic that you want anyway. Just work out a way to regularly survive their attacks or render them harmless, and sit back and enjoy 45 minutes of draw animations. YAWN.