Here is the full list of all 20 Steel Assault achievements.
Kill 100 enemies
Defeat Lt. Karla Schmidt
Complete the tutorial
Defeat Cpt. Baltazar Kline
Defeat Maj. Clayton Harris
Defeat Cols. Nakamoto
Defeat Gen. Magnus Pierce
Finish the Game in Easy Mode
Finish the Game in Normal Mode
Finish the Game in Hard Mode
Finish the Game in Arcade Mode
Complete any level without taking a hit (normal or any higher mode)
Complete any level without picking up any powerups
Parry 100 projectiles with your electric whip
Defeat 100 enemies with punches
Kill 500 enemies
Kill 1000 enemies
Deploy the zipline 500 times
Slide 500 times
Kill 100 enemies while hanging from the zipline