Depends on which version of Ultron.The one which conquered Slorenia with his army was made of primary Adamantium and the only one who stood up to him was Thor, but that one got also enhanced from Scarlet's reality warping powers to the point his energies and durability were enhanced to unknown levels.His decoy which got destroyed from a lightning bolt from Thor after the other Avengers slowed him down was still hella powerful, especially with his flying platform equipped with ionic discharges, missiles, plasma and so.I do think both this versions will knock down Peter sooner or later.His strength durability and fighting skills are good, but he lacks versatility, he can leap very high and fast and showed sometimes, but no flight, energy emission or superspeed makes him weak in this case.