Metal blows vs zerg no matter what he is making. If he makes ultras, get some extra tank support. But dont go all tanks, because they are not mobile enough to counter swarm. If u have enough M&M with tank support u can kill ultras, just micro if they use defilers. SVs with irradiate is, and will allways be, the best counter terran has for guardians, because it doesn't need any extra techs.