He'll become available after completing the Cell Saga.
00. Become Hercule's Apprentice
Skill: Dynamite Kick
This battle is so easy that Hercule is a push over. This battle literally is over after a few seconds.
01. Incredible! Little Senior Students!
Skill: Present for you
This battle starts off with you fighting kid Trunks and Goten. They'll be manageable but don't underestimate them, especially with their long health bars and frequent combos or grabs. Once you dealt enough damage, they'll both transform to Super Saiyan and Hercule will join the fight as well. Concentrate on either Trunks or Goten and reduce his HP to force him to leave. Once one of them leaves, the other will surely follow. Now finish off Hercule to complete the training.
In progress