Super Hero Achievement
Complete all missions on Formidable difficulty
You can start with this difficulty right away and if you can beat it you will also receive "Sidekick" and "Hero" - however, it is highly suggested that you start on Easy and get every other achievement in the game excpet this and "Hero" (so you'll have 850/1000) because you will be able to use the unlocked armors (Silver Centurion seems to be the best) and you will have a better feeling for what your objectives are, plus you won't be worrying about any secondary or hero objectives.
You will need to take your time and be cautious during this difficulty. The number of enemies and rockets is sort of insane and you will die quickly if you charge around or stay in the open too long. There are a few tricks for the boss fights, so if you are stuck, check the forums for help.