So I have 3 mickey heads on all quests besides 769 - 774 quests in proud mode because of the Pot Scorpion heartless. They do a preemptive strike that instantly defeats me and uses up a turn which you need to defeat all enemies in 1 turn for the mickey head. Activating defense boost 6 doesn't seem to work either. Does anyone else have any tricks or strategies for surviving the attacks?

The preemptive strike doesn't actually use up a turn, since you haven't been able to do anything. It's only if they were to kill you in the middle of your turn that it would waste a turn. You should be ok to take the hit, have your pet revive you, and take them out afterward. If you did it that way and completed the quest without getting the 1-turn victory objective, it's possible that you missed an enemy somewhere. (Alternatively, it's a glitch and you'd need to put in a support ticket to have it fixed.)

You're right! I did miss an enemy. How silly of me lol.