The Zerg unit Guardian which is morphed from Mutalisk is superior against Terran micro units (Marine Medic combination).Especially they are designed to kill marines without loss but their disadvantages are their low speed ,high technology necessarity and high cost, but if you can develop this technology at the middle stages of the game you will absolutely control the game.Here the important point is the guardians can not attack air so you must cover your Guardian swarm with anti-air attack units like Mutalisks,Deveourers(high cost) or Hydralisks.Never move your Guardian swarm near to the Goliaths of your enemy and always protect your Guardians the irritate of science vessels.Here we can give you a useful tip;deploy your Guardians seperate from each other so if there is a splash strike you can guarantee other Guardians from certain death.If you do not have enough vespene sources do not try morphing Guardian swarm and if you decide making Guardian swarm do them as fast as you can.