At this stage of the game, you should be junctioning the really strong magics (that is, Ultima, Holy, Full Life, Flare, Meteor) only your crucial stats. These cruicial stats including HP, Str, Mag, Spd. I usually fill the rest up with Firaga, Blizzaga, Stop, Regen. All those mid-level junctions.

If you don't have the uber magics of Ultima, Flare etc. then I recommend the Islands of Hell and Heaven. Put Enc-None on and just walk around for a while pressing X.

Island of Hell: A bit south west of Deling City. It's the long pointy horn shaped island.

Island of Heaven: Easy to locate. Harder to find. North-east of the big Esthar forest. There should be a bunch of little islands with a funny green patchy colour. The Island of Heven is the round looking one.