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