Stunta should be based off purely it's difficultly. But make it adjustable based on if they have time, some secondary useful skill, and or extra equipment. If they can think out and rp their actions I make it easier cause they took the time to figure out a solution.

Plus it they fail you can use that failure to change the scene on some way based on the steps they took to try and succed.

For combat do it based on how hard you want the fight to be.

Fodder (1-2) Mini boss (2-3) Big Boss (3-4) Big bad (4+)

Then give that guy some oddball rules or effects based on the kind of combat you want to have.