Well, that's one way to make friends. Despite Sigma's deviousness and ferocity in this battle. He was fairly outclassed by Ultron's talents. Even with all that, it really didn't matter who would punch harder or move faster than the eye can see, because the real fight was between the Ultron's AI and the Sigma Virus. While the infection and possession was kind of Sigma thing on infecting any machinery within minutes or less to make them under his control, although Ultron's AI was FAR MORE advanced than his. Sigma was certainly ruthless schemer, but nowhere near that level as Ultron. Sigma was the tenacious one, but Ultron was the stronger android, tougher villain, and deadlier viruses out of all the deadly robots throughout the universe. It wasn't long before Ultron, A-Sigma-Lated him.

YouTube References: "Wiz & Boomstick's Screwattack Death-Battle videos"