Parallel Quest #53: "Dangerous Duo! Warriors Never Rest" bonus objective 2

I'm currently going through all of the PQs trying to complete all of their bonus objectives. I seem to be at an impasse with this particular PQ however.

The objectives are as follows:

• Recover 3 Dragon Balls (primary)
• Defeat Broly with Gohan surviving
• Get 7 Dragon Balls

The primary objective is easy. No problems there. Defeating Broly with Gohan alive? Not so much, but for all the wrong reasons. For starters, in this particular PQ Broly is a moron. He's perfectly content to simply fly around the COM players in circles, occasionally firing off some token Ki blasts while getting pummelled by the allies that I can't avoid getting.

In order to get Gohan to appear you need to allow Broly to KO Videl followed by Goten and Trunks (Kid). Videl goes down like a sack of potatoes, but the other two don't. Even with me bringing zero allies and not assisting in any way, at any point in the fight, Broly (usually) barely survives against those two, however he's often so badly injured that Gohan simply has to breathe on him and he's KO'd.

All that I can find to do after Broly is down is blow up some buildings and return the 3 Dragon Balls that make up the primary objective, which promptly ends the PQ.

Which leaves me with my problem: how do I activate the final bonus objective?