Unlock Ragnarok and Ymir for Multi Player Mode:
Finish the game once.
Unlock New Mechs:
Finish the game on campaign mode. The Mechs you get depend on the difficulty as shown below:
Easy: 2 new mechs.
Medium: 1 new mech.
Hard: 1 new mech.
Difficult: 2 new mechs.
Unlock Rampager:
Beat elite without dying once, Rampager is the ultimate mech, it can fly almost forever, and its cloak has an invisible shield that the ragnarok uses, both activate at the same time and last for a very long time, the shiels keeps you from getting hit so your heat does not rise and you stay completely cloaked.
Get Ymir:
You get Ymir the same way as getting Ragnarok,by beating the entire game. Ymir has different weapon choices that some may prefer in battle.