Instructions
Construct sensor towers and missile turrets around the entrances to your base if you are playing as Terran. The missile turrets will detect cloaked units and sensor towers will alert you when a Dark Templar or Warp Prism carrying High Templars enters your territory. Use your Ghosts to EMP the Templars; this will not only destroy the Templar's shields, but will drain the energy of all the High Templars affected by the EMP, rendering them useless.
Morph Overseers and build spore and spine crawlers to detect and kill the Dark Templars roaming your base if you are playing as Zerg. Keep multiple Overseers with your army to detect Dark Templars. Keep your army spread out when they engage against High Templars to mitigate the damage the High Templar's Psionic Storms will inflict. Retreat if your entire army is covered by Psionic Storms. Use Hydralisks and Infestors to snipe High Templars from afar.
Construct multiple Observers and Photon Canons to detect and kill Dark Templars if you are playing as Protoss. Keep Observers with your army to protect it from Dark Templars. Use your High Templars to kill other High Templars with Feedback and to damage and drain the energy of your enemy's High Templars and Psionic Storms.