Complete all 9 floors of Episode 1.
It requires that you play on at least "Don't Hurt Me" difficulty, but its best to play on "Bring 'Em On!" difficulty so you can also net this achievement after finishing all episodes:
Here are links to a map for each level:
Floor 1:
Floor 2:
Floor 3:
Floor 4:
Floor 5:
Floor 6:
Floor 7:
Floor 8:
Floor 9:
These wikia maps are much more useful than the gamefaqs maps, as they show all item/ key/ secrets/ etc.
Here is a brief legend for the more important info:
-Gold "k", Teal "k" are keys (doors will be colored if they require a certain key)
-Teal "g" is a gun, whether it be Gatling or Machine.
-Green square and arrow is starting point
-Gold squares are the exit
-Squares outlined in white are secret doors
-Red "+" are med kits
-Small teal squares are ammo
-Small teal circles are health plates (+10 health)