Here is the full list of all 23 Alien Slayers achievements.
Defeat the boss monster for the first time
Defeat 100 Bosses
Training for the first time
All training completed
Survival success for the first time
Use mutation for the first time
Apply mutations to all monsters in one mission
Acquire Gen1 Combined Weapon for the first time
Acquire Gen2 Combined Weapon for the first time
Reach drone level 5 in one game
Run the game 5 or more times
Acquire all drop items once in a game
Complete the quest for the first time
Clear Normal difficulty without moving
Earn over 5,000 gold in a one game
Kill 9,999 enemies in one game
Have 3 Combined weapons in one game
Restores 999 armor or health in one game
100 games played
Expert Difficulty Cleared