He doesn't have to make her an expert keyblade wielder.
All he had to do was let her protect Sora WHICH WAS THE ENTIRE FREAKING BUILD UP IN THE GAME.
We know as a Princess of Heart she gets BS pure light abilities.
In 1 she reverts Sora from a Heartless to a person without even needing his body, meaning he is just a walking heart.
In 3 she prevents him from going to actual death by holding his heart and body in purgatory, but it's off screen.
Just have kingdom hearts be summoned, the bad guy is about to annihilate Sora, then just have Kairi release all that pure light in her heart and have it fry the bad guy like Ansem at the end of KH1.
Would that have been so hard?