Using king mickey

Answer from: Kingdom Key
Xalidan, the boss in beast's ballroom and cerberus are the only ones i needed king mickey. good luck dude.

Answer from: Xiro Forme
The final boss of any world will allow the use of mickey

Answer from: angelfang
Xalidan yes, and the door in beasts castle were the two i used him in. =)

Answer from: shlaben
Also if you die on shan-yu you get mickey.

Answer from: Namine and Aqua
If you completly defeat the whole game you can go to the disney castle where the cornerstone of light the will be a warp hole and inside the is an enemy in gold armor (from the secret ending) once you defeat it you get king mickey as a summon

or

if you completly defeat the whole game you can go to the disney castle then go to timeless river there is a new dor to the left and help mickey fight a heartless and you'll get the king's charm after the battle

i dont know if this works i was told them by a friend and he said that he found them on a web site but hope this helps

Answer from: keyblader 12
When you lose defeating xaldin in beast castle, choose "i won't give up." then mickey is summoned by sora.