How do I defeat Orwald in the “Blood of the Dragonfather” mission?

I quickly glanced at your save files. In this situation, it's probably already difficult to find a way out. Mutare should ideally have expert-level Earth and Fire magic skills. But I can give you some advice for the future. When starting a scenario or a single-player map, immediately make a starting save and don't delete it until the end. Then, when it's clear which hero will be the main one, and if the author hasn't assigned all the secondary skills at the start and there aren't any huts or other places along the way that grant the necessary skills, save the game and use a cheat code to check: which skills will appear at each level. You can try choosing different branches to find the most suitable one. Record the results in a way that is convenient for you: a notebook, screenshots, or video. For example, here. Don't forget to return to the save you made later! And also, use PhysMiG. Specifically, for standard campaigns.