Instructions
Collect gas early in the game; the units required to defeat a mass hydra attack require large quantities of vespine gas to construct.
Build sentries in tier one, and allow them to collect plenty of energy.
Build a robotics bay and a robotics facility and produce Colossi, which have a longer range than a hydralisk army. They also are capable of killing a hydralisk in only three sweeps of their dual thermal lances. However, you may want to wait until you have established a second base, as it costs 300 units of minerals and 200 units of gas to produce a single Colossus, and you will need several.
Wait for the Zerg player to attack. As the game progresses, Zerg players become weaker relative to their Protoss opponents, so a patient Protoss player can force a Zerg opponent to attack while at a disadvantage.
Respond to the inevitable attack by retreating to a choke point. Place your sentries and your Colossi to the rear, just out of range of the hydra army.
As the hydra army attempts to attack, use the sentries' force-field ability to block off the ramp, and separate each wave of attackers from the rest of the Zerg army.
Use the Colossi to destroy each wave as it attacks.
Counterattack immediately after destroying the Zerg army to prevent it from regenerating. Kill harvesters by walking your Colossi over walls and barriers.