Here is the full list of all 25 Let Them Come achievements.
You killed 100 enemies in a frenzy!
You finished campaign on highest difficulty
100 Challenges Completed
50 Challenges Completed
20 Challenges Completed
5 Challenges Completed
Thank you for your support!
Game plus mode 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!
4 active bonuses at the same time
You discovered all the alien species
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