Well, for one thing he can mindscan using his Encephaloray, he could overpower Mantis with it. I wouldn't know if cloning is standard knowledge of Lobo's powers so I just wanted to ask.
Let's say Ultron mind-reads Lobo, finds out about clonning and then mind-rapes. In character Ultron would attack quite a lot before resorting to that but Lobo would survive those attacks.
Yeah, Lobo is all over the place in stats and defense. In all likeness Ultron would run out of energy before koing him, if he ever manages to clone himself. The killer robot would take a while to ko Thor, and Lobo is probably more resistant and it would be impossible to ko any number of them. He should be able to ko just one Lobo though.