No Pain, No Gain Achievement
No Pain, No Gain10
Kill an enemy with an explosive that also damages you.
You can just this on just about any level where you can find an explosive item such as a gas can, compressed gas cylinders, fire extinguishers etc. Shoot an enemy a few times to lower their health and blow up any explosive item while near them. You should take a small amount of damage where as the explosive item will kill them.
Alternatively you can use the grenade launcher, run up to an enemy and fire a grenade at them. This should instantly kill them and cause a large amount of damage to Matt Hazard.