Tips for 3M damage: Really high combat level would help. 35+.
Play on Easy.
Fight holy foes that are extremly weak to dark. Lower their defence to the make. -50%. Buff A Matt equiped whit fully upgraded Genji armor and Anarchey sword attack (70%). (Note: you should use every stat boost for attack for Matt (Since you are on easy difficulty it shouldn't be a problem) Try Freezing the targect too before using Matt's limit break: Power Blast. It's 70% luck based, but it worked for me and got me 3.4M damage. good luck. (Yeah very terrible English...was in a rush so uhh)
Wow...Just wow... The impossible badge is impossible for real.... Last boss on Battle mode does 120 dmg after every attack, and your healing spell can only heal 100...He has 3k hp and drains your hp..And has a spec that hits 200+.. How is this even possible ?