when you have to destory a speific building, choose a mech with chaff. place the chaff on the top of the building, so that when the enemy attacks, all that firepower heads to that building.
ok in the level "size matters part 1" when it says to blow the gatehouse, dont. instead, find a way to get into a little corridor with one atlas in it. face it, and walk up to the left side towards it's leg. The atlas will then start to dance by itself
Beat the game, you get the Ragnarok, and its seperate configuration Ymir.
Complete the game to unlock the Ragnarok in m/p.
Complete the game, any difficulty, to unlock YMIR.