There are several cheats in Postal that once enabled, will net you this achievement.
This is the only achievement that you can unlock. Cheats disable all other achievements.
All you have to do, is simply type the code on your keyboard.
100% health - healthful
20 fire extinguishers - myteamouse
200 flame thrower rounds - flamenstein
50 more shotgun shells - carrymore
50-100 shot gun shells - shotgun
All items, full health - gimmedat
Fireball gun - sternomat
Grenade and fire extinguisher - lobitfar
Grenades, missiles, heatseekers - explodarama
Invincibility - iamsolame
Less damage - thickskin
Level skip - theresnoplacelikeoz
Mine field bombs - crotchbomb
Missiles and grenades - dawholeenchilada
Missiles, heatseekers - titaniii
Return from dead - hesstillgood
Shotgun shells - breakyosak
Spray cannon - shellfest