If you have the "Wild Wasteland" trait activated, then you can get hold of the "Alien Blaster" (Also seen in Mothership Zeta DLC for FO3 I think), which is pretty powerful.
If you head north east from Horowitz Farm, you should see a UFO floating above the ridge. By here there should be 3 aliens. One of which will have the blaster. I found this after getting the achievement but it was really powerful. It took Cazadors out in one shot and made short work of Deathclaws too (As its a pistol, VATS gives you plenty of shots also). Comes in perfect condition with about 200 ammo.
With this weapon, you can go after bigger enemies and should make the 10000 in no time.
I can only assume that this is classed as an energy weapon though as I have not used it to get the achievement myself. Confirmation would be appreciated if anyone knows! - *** Confirmed by Gaulty315 - Thanks! ***