Also from the wiki, since the above only applies to normal humans and not mages. "Humans utilizing ten Command Spells would also be able to damage Servants. Kirei Kotomine and Kiritsugu Emiya at the peak of their strengths during the Fourth Holy Grail War could defeat Servants, but only when the weakest, Caster and Assassin, are at their most exposed, or when they have special conditions in their favor like surprise or suitability against the opponent."