Make adults seem childish and short-sighted.
Right, so stately Huma Manor's under siege by Dr. Scaravich, his battlesuit, and his army of robot scorpions, and Dad Huma is teaming up with Son Huma and the rest of the PCs to beat them back, and you're worrying about what actual role Dad Huma is going to play in all of this.
So, going with what you've said about the situation, start with what would happen if the PCs weren't there: Dad Huma would fight Dr. Scaravich to a standstill, and the robot scorpions would only have to get lucky once.
Then, filter through that principle in the title to get an idea of how Dad Huma is going to react to the PCs. Dad Huma's not there to uncritically back up Son Huma, he's there to do the things Dad Huma wants to do, because nobody can tell him otherwise. Is he going to treat the bunch of outcasts and bad influences Son Huma is running around with the same way he treated the bunch of outcasts and bad influences he ran around with at that age? Learn their powers, understand their weaknesses, try to make teamwork happen?
Of course not. Things are different now, because reasons. Just try and keep those robot scorpions off him and stay out of his way. Surely you can at least do that.
So now you're at an interesting spot where Dad Huma and Son Huma are fighting for the same things but not in the same ways, and that's liable to cause all kinds of problems.
You don't particularly need to endear Dad Huma to the PCs, just play him honestly. Son Huma is stuck with Dad Huma no matter what happens, and to everyone else Dad Huma is just another adult whose desires conflict with what they want for themselves.
That said, what do you play Dad Huma as doing?
Dad Huma, The Power
One of the tools in your GM toolbox is that you're the central arbiter of the fiction. Moves need to be happening in the fiction in order for the dice to come out and augur the results, and the results must follow from the fiction, so a move like directly engage a threat gives you throttles coming and going.
Could a PC just step up to Dr. Scaravich's battlesuit and engage it? Maybe not - that's a common thing for large singular threats, making them need some work on the PCs part to create a vulnerability before anyone can fight them directly. Maybe so, but when you trade damage you manage to scratch the paint before you get punted into the air and 20-comboed by the point defense lasers.
But when Dad Huma's also fighting it, that changes things. You don't have its full focus so it will do less to you, and you don't need to deal meaningful damage as much as you need to distract it so Dad Huma can win an exchange.
Of course, that might not really establish Dad Huma unless somebody tries to step to Dr. Scaravich when he's not around? But it can still happen.
Dad Huma, The Threat
This isn't to say that suddenly Dad Huma's going to start hunting down Son Huma's team, but more that, if Dad Huma is that strong, and not particularly concerned about how all these interlopers in the fight are doing, the side-effects of his fight can be something you draw from to threaten the PCs. Dad Huma pitches a robot scorpion in a random direction. Dad Huma rams into Dr. Scaravich and the battlesuit goes stumbling back. Dad Huma throws around bursts of blinding light.
Even if Dad Huma's not the sort of person to be deliberately careless, the fight is easily intense enough that he can't be perfectly careful, either.
Dad Huma, The Threatened
I understand not wanting Dad Huma to just get shot by Dr. Scaravich's Petro-Ray. But the reason it probably feels off with you is that it doesn't feel like a fitting way for Dad Huma to get hurt. Dad Huma should get hurt in keeping with his character, and I don't know what that's going to be for you, but one of the primary Legacy strengths is Savior. So, following what a high Savior makes you good at:
Dad Huma doesn't just get shot. Dad Huma gets shot saving you. Fixing something in the manor you accidentally broke, punching through the pile of robo-scorpions that swarmed you, taking the blast from Dr. Scaravich when you finally got his attention and couldn't get out of the way.
The result is the same (you rolled badly, Dad Huma takes it on the chin) but the fiction fits Dad Huma better.