This is it, right?G 80

You'll earn this after destroying the Boss at the end of Area 3. Remember to set the game on Easy difficulty and just keep firing. This should come pretty easily.

At this point if you've been destroying everything on screen you should reach the full 100% completion at this point. The game does continue past Area 3 but is not necessary for any achievements, unless for some reason you are missing any specific enemy kill achievements.