heres what i would do asuming they have lurkers and maybe a few hydras under DS Terran: i would use tank splash, to try to lure them out, if they move then sick'em with what ever you want, if they don't, then send firebats. reveal the burrowed lurkers and blind them then kill what ever else is there so they cant see, they will be useless once hydras are gone. and even just the splash of a tank would kill hydras pretty quick. Protoss: i would use zealots, cost effective, low tech, yet melee and powerful, even late game, just get upgrades for legs and armor and shielding. a nice upgraded squad of zealots would rip them apart. plus if zealots go under the DS then the hydras cant do anything. they can only hit them on the way in, once they're in, the only threat is lurkers or zerg melee. that goes for the terran firebat too. Zerg: your own lurkers burrowed inside the DS, then its just a lurker *****. use queen to kill lurkers then slaughter the hydras with your broodlings, make him feel stupid cuz hydra wont be able to hurt the broodlings basicly just use the dark swarm to you advantage: they can't move or they are vulnerable, melee can still hit them, but if your melee is in the DS with them it protects you too. those are all advantages you have to work with and its more then enough.