Here is the full list of all 21 1917 - there are The Alien Invasion achievements.
Playthrough with less than 10 deaths
Playthrough without death. Become a true Bullet Hell Legend
Get a 1500 Combo
Get a 1000 Combo
Get a 666 Combo
Playthrough with less than 25 deaths
Beat the complete game with the "FleischDecker" (in one sitting)
Beat the complete game with the "SturmDecker" (In one sitting)
Beat the game and unlock the Hard Mode
Playthrough with less than 100 deaths
Playthrough with less than 50 deaths
Kill the true final Boss
Kill the seventh Boss
Kill the sixth Boss
Kill the fith Boss
Kill the fourth Boss
Kill the third Boss
Kill the second Boss
Get a 360 Combo
Kill the first Boss
Unlock the "Luftschraube"