Here is the full list of all 23 Let Them Come achievements worth 1,000 gamerscore.
You killed 100 enemies in a frenzy!
Game plus mode completed!
10 000 Enemies Killed!
100 Challenges Completed.
50 Challenges Completed.
20 Challenges Completed.
5 Challenges Completed.
You completed a wave using only genades.
You completed a wave using only secondary items.
You have come to the end but new challenges await!
You finished campaign on highest difficulty.
Moving On
5000 Enemies Killed!
You discovered all the alien species.
You killed 10 enemies at once!
1000 Enemies Killed!
Perfect combo bonus!
100 Enemies Killed!
You completed the game tutorial.
Set a new challenge highscore.