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