The Mass Mutalisk Build
A lot of Protoss players find it difficult to handle mass mutalisks. In large flocks, mutalisks can be deadly, they provide awesome map control, your ground units can be kited to the graveyard and back, your economy will be harassed, and when you expect it least, a large herd of blood-thirsty zerglings will run rampart in your base. So, how do we stop this?
First of all, we have to scout this build. When it is obvious that the Zerg will attempt to produce mutalisks, you will have to start building a Stargate and create phoenixes as soon as possible. In addition to this, build photon cannons in your mineral lines and in sensitive locations around your base.
The Zerg will usually get 7 to 10 mutalisks and attack your mineral lines. 5 phoenixes will make short work of them, with few if any losses. Once you have held off the initial push, use your remaining phoenix to harass the Zerg base. Keep sniping queens and overlords. Each kill is a blow to their powerful economy.
If they simply stay in their base and mass mutalisks, when you have around 10 phoenixes, go pay them a visit and thin the flock a little.
Meanwhile, on the ground, you should be massing archons and zealots. Archons are awesome mutalisk killers. When you have enough, attack the Zerg. The Zerg will be forced to engage your ground army with the mutalisk cloud. When they do that, it's game over for them. The whole point here is to cause the confrontation. If you leave the Zerg player to their own devices, they will simply take control of the entire map and you will eventually be starved of resources, and you won’t be able to hold out against the harassments forever. Mutalisks are good harassment units, but they are pretty bad at direct combat.
As a side note, you may have noticed that mutalisks have a bounce attack. Each time a mutalisk attacks, it will hit a grand total of three targets. The first target will take full damage from the attack minus armor. The second and third ones will be hit by 2/3 and 1/3 of the initial attack's damage before upgrades. Which means the 3rd bounce will deal very little damage. Upgrading your air armor first will make your phoenix a lot more resilient, because the 1 extra armor nullifies the 3rd bounce.
Another interesting thing about the phoenix is its ability to attack on the run. They don't have to stop moving to attack their targets. This makes them potent harassment units against Zerg. With their superior agility, phoenixes can kill overlords and perform hit and run attacks while avoiding enemy air defences. They are also very good scouts, good map control units, and can provide support for ground units by lifting up dangerous enemy units such as infestors. With decent micromanagement, they can inflict quite a lot of damage and even win you the game by killing all of your opponent's overlords.