So what I'm getting out of this is...
You only want the main hero, hero 1, to be able to go into Overdrive, or Super Mode. When you're in "Super Mode", the hero is stronger, with the possibility of new skills, and a new look.
To get into "Super Mode" or "Overdrive" you need to beat up some monsters, but the points are awarded once you kill the beasts...
This reminds me of the Drive Forms in Kingdom Hearts II except the way you gain points is not by beating up monsters but rather collecting Drive Orbs.
Sounds like a very simplistic Overdrive script that has probably already been done. Have you tried searching for all the available Overdrive scripts at the moment and see if something matches that criteria?