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