If Yyou find that the enemy is going to go mass hydra (this strategy dosent protect against dropping hydras or nydusing them) build about 27 or more dark templars and 6 corsairs, line up all the dts and tell them to hold, now when the enemy attacks kill there overlords and when the hydralisks come they will all splatter when they hit the wall. ive found that this strategy dosent work all well with any other mass attack.

CreepColony's Opinion: Scouts might be better against overlords, but I guess the corsairs get web the hydras and then attack the overlords. Also, reavers are also very good against mass hydras.