How do you get infinite form on Kingdom Hearts 2

Do you mean Final Form? You get it by either defeating the Cerberus cup a certain type of way, changing into any form during your battle with Xigbar, or changing into any form while fighting Siax. I got it all 3 way before but DONT get your hopes up with Xigbar. He's hard, and you'll probibly lose and get really mad like I did. Try it with Siax, because he's easy to beat, and it's that simple. With the Cerberus cup, I only did it once, and it was on beginner, so I honestly have NO idea how to do it again. My friend told me to keep switching to wisdom form then valor form then wisdom then valor, but it only worked once. I cant do it again, because Sora keeps turning into antiform. (AntiSora)