1. You need to go into options and enable the console.
2. use any other cheats you want. a better weapon, and max level on the weapon will help you get the boss down before he can spawn any mobs.
3.press "~" to open the console, enter "Ghost" then press "~" again to close the console
4. fly through all the doors untill you get to the boss door.
5. fly through the boss door, open the console again and enter "Walk"
6. now kill the boss, and finish the level. the achievement should pop right away as long as you didn't hit anything the boss spawned by accident. If you don't get the achievement right away, just retry untill you get a boss that dosen't spawn any other enemies. I cheated in the absurditron 9000 and leveled it to full, it only took a couple of seconds to kill the boss, and the bullets don't really explode, so you don't need to worry about catching something the boss spawned in with splash damage, you can find those cheats here http://tower-of-guns.wikia.com/wiki/Category:Cheats