Challenge Mode
Finish the game once and start a new game to have access to Challenge Mode, you can carry out your weapons, and can get multiplier bolts.
Easy weapon upgrades
Enter an area that features a lot of enemies. Defeat all of them except for one with the weapon to be upgraded. Allow the last enemy to kill you or kill yourself by jumping into a hazard. After respawning, your expended ammunition will be restored and your weapon will retain the previously earned experience.
Infinite Ammo with leveling up
Normally, with the Infinite Ammo cheat turned on, your weapons do not gain EXP. However, if you play with the Infinite Ammo cheat turned off and keep using your weapon until it has no ammo left, you can do a trick to get back up to full ammo for free. Simply turn the Infinite Ammo cheat on once your weapon has zero ammo, and fire the weapon a number of times equal to that weapon's maximum ammo count. So, if using the Fusion Grenade, fire it seventeen times with the Infinite Ammo cheat turned on, and then turn it back off. Your weapon will be back up to full ammo, and you can then use it to gain more EXP. This is very helpful in Challenge mode.