Here is the full list of all 20 Alien Hallway achievements.
Complete the game on extreme level of difficulty
Upgrade one unit till 10th level
Win the battle with lower than 50% health level of your base
Get 5 stars for all game levels
Upgrade all friendly units till 4th level
Complete all game levels
Complete any level of the last planet without loss of friendly units
Call for 5 Heavy laser gunners during the battle
Upgrade the base till 5th level
Upgrade airstrike till 7th level
Have 10 Snipers simultaneously present during the battle
Use airstrike 100 times
Liberate the first planet
Throw 300 grenades
Kill 1000 aliens
Accumulate 1000 energy units during the battle (can't be achieved on "Resources refill" and "Final Step" levels)
Kill 100 aliens
Call for 50 friendly units
Call for 20 Flamethrowers during the battle
Call for 20 friendly units during the battle