Hydralisks are one of the most versatile creatures in the game. A cheap cost for easy production, a respectable range for thrashing opponents from afar, a nice 10 points of explosive damage plus the ability to burrow and regenerate. For 75 minerals and 25 gas, indeed the hydra is a must for a zerg war machine.

Hydralisks work best in great number of 10 and above. The grooved spines and muscular upgrades are a must for greater killing capacity. Prioritize the damage upgrades at the evolution chamber over the carapace. Refrain from attack with the hydralisks alone. Be sure the opponent is engaged first with zerglings and ultralisks to buy time for these vicious killers to position and start wreaking havoc.

A good combo is casting dark swarm and hiding your hyras under it. This way they can fire outside and at the same time be protected from long range attacks. This is particularly effective versus terran. Be sure to keep a few zerglings to engange melee units that go inside the dark swarm.

In island games, a famous technique is the hydra drop. To execute this, you must use double lair and upgrade pneumatized carapace and ventral sacs for the overlord. Collect around 12 hydras and a few zerglings and load them into your overlords. Drop them near the enemy's mining operation.