I heard that you get the Final Form after defeating Roxas. But when I defeated him, that drive form didn't get obtained. So how do I unlock it?

After you defeated Roxas the Final Form appears randomly when you use any Drive Form. If it appeared you can use it any time.

Either go into a drive form anywhere and hope you get it, or use Two Become One and go into a form.

 

Another way is to go to Olympus Coliseum and enter the Cerberus Cup, then go into a form. If you don't get Final Form, simply restart the cup.

The keyblade two becomes one actually increases the odds of getting final form or anti form so use that

Go to Olympus Colisseum.

 

Equip Two Become One Keyblade. This Keyblade triggers antiform every time you go into a drive.

Final Form appears randomly instead of antifirom. Therefore, Two Become One increases the chances for Final form as you can't go into any regular forms.

Also, every time you don't get final form but antiform, the chances for final form are increased, if this happens several times it even increases to 100%.

 

Before Two Become One, people had to go into drive forms repeatedly until antiform kicked in. As it took several tries to get anti it took longer to randomly encounter final form as adeviation of antiform.

 

 

To speed up the process even more, enter the Cerberus cup (or the one tha lets you use drive forms)

from there, you just activate any drive form and it should trigger antiform because of Two Become One. Wait until it wears off and activate another drive andantiform should kcik in again.If you run out of bars, just hit enemies to rapidly reload your drive gauge.

 

Repeat this until final form appears instead of anti form.

 

Once you enter final form once, you can activate final form any time form the drive menu like any of the other forms.

 

Note: if you still have Two Become One equipped, the only form you an activate correctly will be final form as all other forms will trigger antiform. This is a certain way to trigger antiform at will but if you want to access all other drive forms, use any other keyblade.

Hope this helps.

You can find more information on anti form and the dark points that trigger it, check khwiki, it'S easy to understand.

(basically, each form stocks up dark points and the more you assemble, the more likely antiform will kick in. Final Form deducts from these points, however, so you can manage them if you keep track)

You can just do it at TWTNW. Just go into a drive in the place you spawn after defeating Roxas, after equipping Two Become One, if you don't get it, just enter the next area you go to, since your party members are just walking around and Sora is alone, your drive gauge will return to max. Then go back to the area with the skyscraper aka the area where you spawn after defeating Roxas, and try it again (gotta be in battle of course). Should take a max of 5 tries. 

Okay then just go into master form but instead of master form final form shows up.

Spam master form in battle at TWTNW, and hope the RNG goes easy on you.

 

 

Equip Two Become One Keyblade. This Keyblade triggers antiform every time you go into a drive.

 

Didn't know that! Imagine my rage when I got pwned by Xigbar almost three times in a row because i "randomly" entered Anti-Form each time.