There are 2 modes in this game
Survivor - In survivor mode, you will get less resources and ammo. No retry is given back when the game saves. You MUST play the entire game in this mode to unlock certain achievements and score bonuses (For Leaderboards)
Normal - In Normal mode, you will get more resources and ammo. Every time the game saves, you will be guaranteed to have a minimum of 3 retries. In this mode, certain Achievements and score bonuses will not unlock.
Beating the game unlocks this achievement as soon as the final cutscene starts.