Step 1: Enter any Travis Deathmatch battle (except Charlie McDonald), I recommend Letz Shake, but it can be anyone, because of the following steps.

Step 2: Run out of battery, just do running slashes until you don't have any battery left

Step 3: Let yourself get killed

Step 4: When the game asks you to retry again, go to the Deathmatch menu or to quit, select Quit, now you'll be on the Start screen

Step 5: Enter again Deathmatch and select the boss you want, now you have unlimited battery with the weapon that you had equipped at the beggining of the match.