To unlock Cheat Mode, beat the game with a 100% game completion.
Ragnarok Assault Mech in Multiplayer mode:
In the “Divergent Entry” mission, climb the wall to access the second area. Look right to find a locked bunker. Use the access terminal on the left side of that bunker to play the hacking mini-game. Beat the game to unlock the bunker, which contains the Ragnarok Assault Mech. Use the mech to complete the level to unlock it in Multiplayer mode.