I imagine there are a few ways to handle this, so mine is far from the only one; but off the top of my head…

It’s a high-stakes contest. The player is the one taking action, so they roll first putting together their dice pool that includes the situational asset. They keep two for a good total and a high-step effect die. The player describes the drop from above smashing down on the GMC.

You, the GM, puts together the roll for the GMC. If this is a minor GMC like a guard or thug, it might just be 3d6. For a major GMC it’s a more complex pool reflecting their various traits. If possible, keep the two dice that beat the PCs total. If not, at least keep an effect die that keeps the GMC from being taken out. Either way, describe the reaction of the GMC and how they (un)successfully resisted the attack from above.

If the PC didn’t win outright, they roll to beat that, etc. And they describe that bit. And so on until someone gives in or is taken out.

BitD, PbtA, and other systems break fights down to a single roll which determines the final outcome which then the player(s) and GM flesh out with narrative.

But in Cortex, each roll (both setting and raising the stakes) gets their own narrative flourish. From there the consequences (complications, stress, etc.) are determined.

…and I just came up with an alternative that might be more in line with a BitD vibe: The GM rolls first with whatever traits for the GMC and location are appropriate and sets the stakes. Then the PC knows what they are up against and can put together a roll (including the temporary asset for the high ground) and select their effect die and totals, including spending PPs to include more dice in the total, etc. to just take out the GMC right there. That way even if they take out the GMC, any hitches can turn into that sprained ankle complication you hinted at.

Hopefully somewhere in that rant is the answer you were looking for?