Here is the full list of all 14 Megaton Rainfall achievements.
Finish one mission with no casualties
Acquire every superpower
Get every trophy
Beat the game
Beat the game in hard mode
Beat the game in less than two hours
Finish 3 missions with no casualties
Get rank A in one mission
Get rank A in all the missions
Destroy 3 enemies with a single Gigaton Blast
Destroy Earth
Kill yourself
Collect every Xenosphere
Get really far from Earth