Hello. Other people have already reported the issue you're describing, and I'm actually releasing version 4.1 today to fix it. The stomp now has a damage setting so it respects each enemy's HP (tough enemies take several jumps), you can mark which enemies can or can't be stomped (spiky ones hurt you), and you can let the boss take stomps too — exactly the "really fight by jumping" you're after.
As for custom screens, that's a feature I've reserved for the paid offline version, which is what helps me keep improving the project. Thanks a lot for the feedback. 🎮