Hidden Conditions:
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You have to defeat Yamcha, Tien and Krillin, and finally Vegeta while keeping Hercule's HP above 50% to trigger the hidden conditions. This should be pretty manageable if your character is already strong enough and if you brought powerful support characters. After defeating Vegeta, he should transform to a Super Saiyan. Defeat him to complete the quest.
Have tried several times. Hercule is at about 90%. Will not trigger.
Doing it, but can't unlock the hidden requirements despite having Hercules heath above 50>
Have you tried using a Super or Ultimate Attack as a finisher and/or beating the quest within 5 or 10 minutes? Most of the times, that'll do the trick