I personally think that level 99 is overkill for datas. You wouldn't need to rely much on strategy then.
Xemnas, Xigbar, Lexeaus, Zexion, Saix, Axel, Demyx, Luxord, Marluxia, Larxene are very doable even in level 1 because of how much you can exploit from them. Great way to farm stat ups as well.
Farm Guard Ups with Saix, summon Peter Pan at the very start, jump and double combo, repeat. He can't even land a single move on you till the end.
Farm Magic Ups with Axel, reflect + full land combo + hi-potion, do his first Reaction Command only, repeat. And reflect throughout his Desperation Move.
Farm Larxene for Lost Illusions. Reflega + Final Form Genie/Stitch her till the screen turns dark, use Donald's Comet and watch her die.
Farm AP Ups with Luxord. Demyx is easier but longer. Skip all his games with a reflect in his face then combo. Glide away till he starts a new game and repeat. When he turn into cards, flip the clock card first then him and combo. When he has around 20% time gauge left, use Trinity Limit, elixer, follow up Trinity Limit, and with your restored MP, Trinity Limit again, elixer, follow up....till he dies. Won't even need to play his final game if you have bad timing. I think Limit Form's Ether+Ars Arcanum spam prevents his final game as well. Though you should really play it if you are good, since winning gets his HP to 1.
I prefer synthesizing Power Ups since I don't use my materials for the above. But for Xemnas first form, guard his attack and full combo, repeat. When he run up the building, use the third Reaction Command and finish him. Final form, dodge his attack and slide up to him and reflect/reflect+combo. He grabs you, enter Session. Guard his lasers and combo when he finished the chain of attacks, Session if necessary.
Personally, I have a tougher time facing the Cavern of Remembrance at low levels than the Data battles......