In Megaton, go to crater side supply with at least 2 frag mines (second one's just backup).

Get on the other side of the wall behind that merc who's always hanging around there.

Crouch down (makes it a lot easier) and throw a frag mine down.

It will immidiately begin it's countdown beeping to explode. Disarm it. Exp gained.

Another method is to simply put the game on very easy and RIGHT BEFORE the enemy(s) die, put the game on very hard.

This works amazingly with enclave, death claws, and super mutant masters/overlords.