There is an extremely simple way to garner this achievement, with the only requirement being to play on Recruit difficulty until you reach the point where Imps begin to spawn.
While playing on Recruit difficulty, find a place where there is a lone Imp in a room with you. The Imp's fireball will do 3 points of damage to you, while their melee attack (claw swipe) will do 2 points of damage. Get in as close as you possibly can to them, and allow them to attack you. As long as you are within claw range, the only attack they will use will be their claws, thus taking you down 2 health points per attack. As long as your health ends in an odd number, they will eventually knock you down to 1 hp. At this point, simply retreat out of their reach, and dodge their fireballs while shooting them. Before long, they will die and the achievement will be yours.
If your health ends in an even number while doing this, briefly duck out the Imp's melee range (back up 2-3 steps), and allow them to hit you with a single fireball. This will drop your health just enough so that it now ends in an odd number, and you can proceed as noted above.
If it helps, when I did this I was on the Enpro level, in the room immediately after finding the Plasma Rifle in the locked cabinet in the office. This was also when I realized the amount of hp they were doing with each attack, so it could have been done much earlier in the game.