Hmm, I have an idea for yet another very-very strong attack.
Here goes:
Comet2 attacks enemies multiple times based on how strong the user's magic rate is. If you use heaps of magic source on the player, Magic increases (Up to 255 max I think).
Now say each Comet2 hits 6 times, dealing 3,000 per hit (because of strong magic rate). Thats already like Quadra-Magic with something strong (ie Ultima). So lets say Comet2 on its own does 18,000 (6hits*3,000=18,000).
This is the plan (everything at max level) for a 14-slot setup:
Comet2 + QuadraMagic (Makes Comet2 be cast four times later)
Comet2 + MagicCounter (Comet2 four times when hit)
Counter + Mime (Comet2 another four times when hit)
Counter + Mime (Comet2 another four times when hit)
Counter + Mime (Comet2 another four times when hit)
Counter + Mime (Comet2 another four times when hit)
Life2 + FinalAttack (Good to keep it there just incase)
This equals up to 20 Comet2 castings. Which equals 360,000 damage every time you are hit!
Of course this depends on how strong the players magic is, and on how weak the targets spirit is (And immunity to non-elemental attacks). But even so, this thing can do a LOT of damage.
Also keep in mind, QuadraMagic does not cost any extra mp, so all up, it would cost the same as 5 Comet2 castings - 110*5 = 550mp per counterattack.
If thats too much, you may consider swapping a Counter+Mime with a Comet2+MpAbsorb (absorbs 1/100th of damage I think - 210mp per cast), or adding in HP<->MP Materia (And shielding the player during battle, he can only have max 999hp - Just put cover on someone).
Anyway, that pretty much covers my idea of another outrageously strong attack.
-Formerly DarkSephiroth-