Beat the game with 100% completion to unlock cheats after the credits.
Ragnarok Assault Mech in multiplayer:
In the mission 'Divergent Entry', climb the wall into the second area. On the right you should see a locked storage bunker. Use the terminal on the left to play a hacking mini-game, which will open the door to the Ragnarok. Use it to finish the rest of the level to unlock it in multiplayer mode.
Thanks to Revolution Reader Spheroid!