One idea could be a story where Mash teams up with an unlikely ally from a rival school. They start off not getting along but through various challenges, they learn to work together and combine their unique skills.
Another option could be a student from a different magic school who is jealous of Mash's popularity and unique abilities. This student could use dark magic to try and bring Mash down.
Start with understanding the basic anatomy of muscles. Study how they attach and move. Then, practice simple sketches to get the shape and proportions right.