Go to the main menu and select the "new game" option. Hold L+R+Z and quickly press left, up, down, A, right, B, start to enable the cheat entry screen. There it's possible to enter one of the following codes:
Level select: WEWILLWIN
Invincibility: SUPERMECH
All multi-player levels: MULTIMAYHAM
All paint schemes: MISSMACROSS
All veritechs and medals: WHERESMAX
Both gunpods and missiles refresh quicker: MIRIYA
Disable active cheats: CLEAR
1 Shot Kills: BACKSTABBER
1 Shot Sniper Kills: SNIPER
Upside Down: FLIPSIDE
Fast Gunpod Refresh: SPACEFOLD
Fast Missile Refresh: MARSBASE