For completing any of the eight the Arena mode levels with an A rank or higher, you can unlock cheats in the Classified section on the main menu
Effect - Code
EX Difficulty - Get an A rank or higher on arena level 1
Guns never overheat - Get an A rank or higher on arena level 2
Have All Guns and Gun Upgrades - Get an A rank or higher on arena level 4
Health Recharges Automatically - Get an A rank or higher on arena level 6
Protocol Recharges Automatically - Get an A rank or higher on arena level 5
Shield Never Overheats - Get an A rank or higher on arena level 3
White Texture Mode - Get an A rank or higher on arena level 8
Wireframe Mode - Get an A rank or higher on arena level 7