Beat the game once With 100% completion.
In the mission "Divergent Entry," you must climb a wall to access the second area. To your right, you will see a closed bunker. On the left side of this bunker is an access terminal. Beat the hacking minigame to open the bunker. Inside, you will find the Ragnarok Assault Mech. Beat the level in this mech to unlock it for multiplayer use.