This Zerg player has no anti-air defenses making this setup ripe for a Cloaked Wraith attack. The Terran Valkyrie is a fairly recent addition to the Terran air force, and a rather unwelcome one from the Zerg perspective. Their cluster missiles damage all air units in a target area and can quickly annihilate groups of Overlords or Mutalisks with a minimum of effort. Fortunately Valkyries are slow moving and make prime targets for Scourge and Hydralisks. Hydralisks have nothing to fear from the Valkyrie since the Valkyrie has no ability to attack ground unit.
Scourge can strike Valkyries before they have a chance to unleash their missile volleys.
Irradiate Irradiate is one of the deadliest threats to the Zerg. Tightly grouped or stacked groups of Hydralisks, Zerglings, Mutalisks, or Guardians take heavy damage from an Irradiate. Whenever a Science Vessel is spotted, destroy the blasted thing immediately with Scourge. Should your forces encounter a Science Vessel, try to spread them out to reduce the effects of Irradiate. Equip your Hydralisk horde with Burrowing to help combat Irradiate.