I am trying to complete the challenge "Demise of the Machines" to get the perk for more damage against robots, but cannot find any living robots. I have gone to every location in the Wasteland and killed pretty much everything, so I was hoping maybe there was a place where I could find some robots that respawn.

There's a respawning mister gutsy prototype in Nipton, for what it's worth.

Right on, thanks! I'm at like 39/50 robots killed, so even one respawning robot is good enough

Much like Super Mutant Massacre this perk become meaningless in achieving it since there aren't many more than 50 robot enemies in the game anyway. Once you've killed the Securitrons on the Strip and all robots at Helios One, Vault 11, the weather station, Repconn Offices, the crashed Vertibird and the turrets at Searchlight there's only ED-E and Fisto, and they're on your side.

Yeah, I know it is kinda pointless to have, but I figure whenever the new DLC comes out set in the Big MT, there will probably be a bunch of robots, so I want the damage bonus.