Ive synthesized the ultima weapon before taking on the final boss on Critical just as I've done in Kingdom Hearts Final Mix on Proud. Haven't played KH2 in awhile and this i think is the first time I actually got the ultima weapon and the first time I will beat the game on the hardest difficulty. After the final boss going to take Mushroom XIII, Data Battles of Organization XIII, and Lingering Will which will provide a real challenge.

Awesome, good for you!  Now go forward and beat the game, and then complete the rest of the challenges the game has for you!  Trust me, The Lingering Will is a very serious threat, but you can get the job done!