Meh. There's no real perfect answer. I basically followed the Ice path for Serph, with a secondary path for the Hunt skills - both of these he starts off at. Argilla went Earth and Healing spells, Heat went almost entirely for physical attack skills with a sideline into Fire. Gale went Wind and Death, Cielo Lightning and whatever I needed at the time.
Important safety tip, you must get _everyone_ the Mantra that's got the Mutual Karma skill in it as soon as possible. Less annoying leveling that way.
When you get far enough (max out an elemental path - Serph came that far in my run through) start getting the newly available Almighty stuff, for the simple reason that somewhere up there is a nifty little spell called Magic Repel, as well as the Null Element passive which basically makes that character your party's magic shield/enemy turn canceller. You can use the Almighty stuff if you want to, but frankly, it's neither neccessary nor is it very effective. You want magic damage, get a -dyne level Elemental spell with both Amp and Boost passives for that element picked.
-Griever