Well though luck, but you can Lower your chance of getting into Anti form with this neat thing.
Taken from A Gamefaqs guide from Fayt2700
I've recieved some E-mails regarding the prevention and how often you'll
transform into this form, and it's all based on a point system. The
following was provided by Beamneocube:
There actually is a pattern/probability of changing into Anti-form.
There are these things that you cant see called Anti-points. Whenever
you go into Valor, Wisdom, or Master forms, your Anti-points go up by
one (+1). If you change form while an ally is unconcious, the points do
nothing (+0, -0) When you change into Final form, the Anti-points go
down by ten(-10). Whenever you actually go into Anti-Form, they drop by
four (-4). Also, when you aquire a new form, the points reset
themselves. So basically, the more Anti-points you have, the more of a
chance there is that you will go into Anti Form. If you have 0-4 Anti
points, there is a 0% chance of changing. If you have 5-9 points, there
is a 10% chance. If you have 10 or more, then there is a 25% chance.
That though is not the only thing that affects the change rate. During a
normal fight, the percent is x1. During most scripted battles,
the chance is x2. During fights with Organization XIII, it is a x4.
Against Armored Xemnas, there is a x10 multiplier. In battles with
Non-Party Coopritive Characters, (such as those with the people from
Final Fantasy and when Hercules is with you, but you cant see his health
bar) there is no chance of transforming (x0).
So how does one prevent Anti-Sora? Simple, use the hell out of Final,
which will knock down the Anti-Points by a total of 10, meaning the
more you use it, the less of a chance you'll have of EVER becoming
Anti-Sora again.Do for Demyx you will have a x4 multiplier of getting into anti form UNLESS you have less than 4 anti points.