Here is the full list of all 25 Let Them Come achievements.
You discovered all the alien species
Moving on
5000 Enemies Killed!
You killed 10 enemies at once!
2 active bonuses at the same time
1000 Enemies Killed!
Perfect combo bonus!
100 Enemies Killed!
You completed the game tutorial
Thank you for your support!
You have come to the end but new challenges await!
10 000 Enemies Killed!
5 Challenges Completed
20 Challenges Completed
50 Challenges Completed
100 Challenges Completed
You completed a wave using only secondary items
You completed a wave using only genades
You killed 100 enemies in a frenzy!
You finished campaign on highest difficulty
4 active bonuses at the same time
Game plus mode completed