Taking damage does not lower enmity.
You can test this yourself by using the skill 'Provoke" on an enemy and having a friend "Provoke" as well. Provoke only generates 1 point of enmity and can easily be lost by simply having another party member buff themselves.

The only way to 'lose' enmity is to die... where you restart to 0 (and hence your only hope of regaining the enmity back is the skill "provoke").

There is however... some weird WS mechanic on certain monsters. Let's say you use provoke on an enemy that's performing an instant weaponskill with no timer (such as a simple cleave attack). Even though your highest on the enmity list during their WS initiation time, their previous target is already marked to receive the attack. Once this attack is done, only then will the enemy come back to you. It's somewhat annoying when this happens and the only way to stop this is to interrupt the enemy via stun/silence.