You need to be playing as Vegeta to begin this substory, which is inside of Capsule Corporation, at the end of the hall by the researcher. Note that Goku, Goten and Trunks (Kid) cannot be in your party if you wish to do this. Trunks is acting suspicious, so he says that he saw a strong-looking warrior on the outskirts of West City. Exit Capsule Corp and head for the blue marker, where you will find Goku waiting for you. It turns out the strong warrior was actually Goten and Trunks, who fuse into Gotenks (Super Saiyan 3, too) and ask for a fight between you, with Goku as a support character.

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(1 of 2) Galactic Donuts will completely stop you from doing anything for a few seconds

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Galactic Donuts will completely stop you from doing anything for a few seconds (left), Big Bang Attack will be Vegeta’s best move due to its damage (right)

Gotenks here is not any different than when you briefly controlled him during the main story. It appears Gotenks only uses two moves here, Galactic Donuts and Super Ghost Kamikaze Attack. The former is a projectile in the shape of a donut and should it hit you, then you will be unable to do anything for a few seconds. It may look like the attack will go over your head, but the hitbox is a bit deceptive, so dodge or block. The Ghost Kamikaze Attack is definitely the worst of the two, as the ghosts summoned will fly around the arena and when you think it’s safe, you’ll end up getting hit by one. There’s nothing really concrete that will help in avoid it, too. Considering it’s a two-on-one, in your favor, this battle isn’t very hard at all.