Successfully complete the game with a 100% completion to unlock the cheats after the credits.

In the "Divergent Entry" mission, climb the wall to access the second area. Look to the right to find a locked bunker. Use the access terminal on the left side of that bunker to play the hacking mini-game. Successfully complete the game to unlock the bunker, which contains the Ragnarok Assault Mech. Use that mech to complete the level to unlock it in Multiplayer mode.