Save an early pistol for this achievement. If you don't have a low-level pistol, return to the starting area on the first playthough. The chest on top of the gas station should have a level 1 or 2 pistol.
Find a pack of level 5-ish skrags, shoot one a couple of times with the low-level pistol to reduce its health to single digits, then jump on that skrag. Jumping deals a small amount of damage. Kill the enemy with this damage to unlock the achievement.