Here is the full list of all 22 Alien Hallway 2 achievements.
Kill 1000 aliens.
Kill 50000 aliens.
Kill 500000 aliens.
Collect 5000 crystals.
Collect 50000 crystals.
Collect 500000 crystals.
Kill 1000 aliens by barrel explode.
Complite 10 additional missions.
Complite 50 additional missions.
Earn 25 stars.
Earn 50 stars.
Collect 30000 coins.
Collect 100000 coins.
Collect 3500000 coins.
Kill 20 bosses on easy level.
Kill 20 bosses on hard level.
Kill boss Kraken on hard level.
Upgrade any soldiers 10 times
Upgrade soldier to maximum.
Upgrade all soldiers to maximum.
Destroy 500 small boxes
Destroy 1000 big boxes.