It's easier to use hydras, since generally, terran playing bc's aren't the least bit worried thier base will be attacked, mainly because they have an allie for support. Use Plague on the bc's, then send hydras in to finish the job.
But, if you are feeling ill confident in the abilitys of 12-15 bcs, with 250-300 hp vs 80 hydras or so, then send in scourages to clean them out, then charge at thier base with hydras. BC's take too long to build, wraiths are ill equipped to battle a hydra, and they obviously based their offense on air, so they shouldn't be too complicated.
Another good zerg strat oftenly used by me, is too make several Queen's early-hand, so you can ensnare, which greatly reduces speed of movment and firing, so you can then simple mass them with scourges or hydras.
If your Terran, best to go lockdown, and/or Valk.
If your Protoss, best to use corsair till thier weakened, then use statis field so you can kill them while thier frozen for several minutes.
Gemini_Bloodian