I'm at the end of KH2, playing on standard mode, and just got to the part where I'm fighting Xemnas in the black and white robe. I can't beat him though. I'm level 61 and had no difficulties beating any of the bosses up to this point. In fact I thought the other members of Organization XIII were ridiculously easy. But I can't get past the part at the beginning of this fight, where I'm tossed in the air and being attacked. I can do reverses but those don't even protect me. My question is: Is this battle optional? Is there a way to get out of it or do I have to beat him. I don't remember this battle on KH2 for the ps2 but I did beat the game on the ps2 (I remember the credits and the ending scenes where Sora and Riku are back on the island and receive the message in the bottle). I'm afraid to just hit reload when I die instead of continue because I really don't want to fight all those battles over again. So I figured I'd ask. Can I quit and level up some more without having to beat everything all over again?

If you go back to level, you have to do all four parts of the boss fight again, you can't just skip to the final fight. And yes, it was in the original KH2, the only boss fights they added were Roxas, data fights, and absent silhouettes. And no, it's not optional, you have to complete it. All you can really do is get to the ground and heal if you need do, then wait for the few openings he gives and attack.

When I was playing Kingdom Hearts 2 Final Mix I was on Proud mode and I beat Xemnas on level 52. The final battle was very easy because I use Thundaga, Reflega, and Curaga with a bunch of Hi Potions. The battle is not optional so you have to beat Xemnas no matter what in order to complete the game. Like Azure Flame said you can't skip the final battle. 

Final Xemnas is a catastrophe in 2.5. The move where you get throw in the air doesn't work the way it is supposed to in 2.5. Only way to get around it is to use Session. Good luck

I just spammed reversal and healed when I got the chance. The Leaf Bracer ability helps a lot.

Yeah, use the Session Limit whenever he uses that attack, as Skhyatze said. You're immune to damage during it.

what is the session limit? How do I activate it? If I have to reload and do it over again I wanna make sure I have everything I need this time around. Do I need to level up some more? I managed to get past that first part and it was really easy to deplete Xemnas' hp but those red lasers and his blasts took me out in no time. What is the recommended level to be at (that's realistic, I'm not looking at level 99 or anything, I just wanna beat the game) for standard mode? What limits, besides session, should I have active? I do have leaf bracer and once more. Should I be using thundaga? What's the best method to beat him? It must've been easier on the PS2 because I really don't remember this fight. 

yea in 2.5 the reaction command to dodge Xemnas' attacks is bugged so you're better off activating Riku and Sora's Session Limit. In combat hit left on the D-Pad and the 4th command should be Limits. "Session" will be there as long as it's activated in the abilities menu.