Onion Knight ability:

Break Damage Limit:
Allow Lulu to do more than 9999 damage
Triple Overdrive:
Lulu's Overdrive charge three times more faster than normal.
Magic Booster:
Double spell damage but it'll also double the MP cost.
One MP Cost:
All Spell Cost are reduced to 1 MP.

So, Lulu can cast any magic with 2 MP (Even Ultima and Flare).