-Just go in infinite mode on normal and DO NOT touch the screen the whole time.
-Let your ship killing of the enemies.
-When the Game is over, just do the same, go in infinite mode on normal rinse and repeat.
-Do this 30- 45 min, it can varius......
.... and then the Game starts with only 1 or 2 enemies fly to you and after this the Game is empty no more enemy spawn you flight alone with your ship in the space.
Let it 3 hours flighing and your Achievement unlocked.
Make sure no one rings you up, or something else. I dont know what happend then.