Activating this trainer:
Press F1 at main menu.
Listen for 'Trainer Activated'.
Press desired hotkey. All hotkeys can be changed on the trainer.
NumPad1: Infinite Health
NumPad2: Change Score By 5000
NumPad3: Unlimited Ammo
NumPad4: Unlock Characters
NumPad5: Unlimited Missiles
Unlock Characters: While active you can select any character as they are unlocked. This is only temporarily means if you deactivate this option the characters are locked again (except you unlocked them already).
Unlock more trainer options including updates from here - Bullshot Trainer