Stock up on Staminan Royales
Get bullet proof vests or chain mail
Get steel shin guards
Keep upgrading max health and attack power
Get weapons like guns and blades
Fights are basically all about out-healing your opponent so as long as you can get hits in and continuously heal, you’ll eventually win.
It’s not like you have to fight right there. You have free roam to keep getting stronger and buy more medicine items and weapons