Here is the full list of all 31 STHELL achievements.
Destroy 100 Breachtrons Model V-004H
Destroy 10 Cargo Mechas Model D-808E
Destroy 50 Pathfinder Model P-003E
Destroy 50 Rollers Model K-4M1KZ
Destroy 50 Drones Model F-284G
Destroy 30 Bipeds Model V-107Q
Destroy 50 Breachtrons Model V-104H
Destroy 25 Houndtrons Model B-55H
Destroy 200 Drones Model K-88A out of a swarm
Destroy 5 Mobile Barriers Model B-086E
Destroy 10 Spidertrons Model S-682T
Destroy 15 Bipeds Model V-655R
Destroy The Monolith Prototype Zero
Destroy the Mantis Prototype Zero
Destroy the Spider Sentinel Model S-900T
Destroy The Tripod
Get the Shotgun
Get the Brute
Get the Revolver
Get the Energy Gun
Get the Heavy Machine Gun
Get the PPR
Die for the first time
Die 25 times
Die 50 times
Stay alive for 5 minutes
Stay alive for 10 minutes
Defeat the first boss using the Semi Automatic Pistol only
Defeat the second boss using the Semi Automatic Pistol only
Defeat the third boss using the Shotgun only
Survive 3 minutes with your health under 30