The Hydralisk is a versatile Zerg ranged unit that can attack both air and ground targets using needle spines. Hydralisks can be morphed from Larvae after building a Hydralisk Den. Hydralisks deal explosive damage, which makes them good against buildings and large units.
They are a medium unit with varying counters based upon the fact that they are medium-ranged, fairly fast-moving (once speed-upgraded), and deal explosive damage. They are used in all three match-ups in varying ways, but less so in ZvZ due to their explosive damage type that is ineffective against small units. They have few other special traits except being very efficient for supply, even performing admirably against many extreme late-game compositions. For one supply they have double the health of a Marine; they still tend not to be too cost-efficient, however.
When using Hydralisks, it is important to research both the Muscular Augments and Grooved Spines upgrades from the Hydralisk Den as soon as possible for increased speed and range, respectively. These upgrades greatly increase the capacity for Hydralisk micro and effectiveness.
Against Terran, Hydralisks see varying amounts of usage. When faced against a bionic Terran army, Hydralisk useage depends on a few factors. Hydralisks and Lurkers are the ideal late game unit combination to fight the SK Terran strategy, in combination with Defilers. Marines and Medics themselves do quite well against Hydralisks, which is why they are rarely seen until the late game. However, any bionic army that incorporates significant numbers of Tanks will generally obliterate Hydralisks, in which case they are only built to morph into Lurkers.
However, against a mechanized Terran army consisting of Goliaths, Tanks, and Vultures, Hydralisks are combined with Mutalisks to counter the Terran mech ball. Hydralisks are a cost-effective counter to Vultures and Goliaths, while Mutalisks can snipe stray Tanks. Since Hydralisks fall easily against a large enough mechanized Terran force, they are usually used in Drops on the Terran Main late game and are combined with the Defiler’s Plague ability to quickly decimate a Terran mech force. Hydralisks are also used to defuse mines more easily.
Against the 2 Port Wraith build, Hydralisks are used to snipe enemy Wraiths and prevent them from doing too much harassment. Since the preferred transition from 2 Port Wraiths is a bionic army, the midgame Zerg army consists of Hydralisks and Lurkers until Zergling upgrades and Defiler tech is finished.
When Terran techs to Vessels while playing Bionic, Zerg often opts for adding a few Hydralisks to his army to snipe plagued Science Vessels.
In ZvZ, Hydralisks are not often seen. The Hydralisk's explosive damage is reduced by half against Mutalisks and Zerglings, two units most commonly seen in this matchup, so they are weak before they can be stacked up and must be supplemented in the early game by base defense and massed quickly enough before Mutalisk which also en masse are very powerful. Hydralisks also lack mobility compared to these two units, making them unwieldly for defending multiple bases. However, there are certain builds that utilize sufficient numbers of Hydralisks to push towards the opponent’s base in mid-game timing-based attacks meant to give the Hydralisk player enough army lead to expand safely.
In the rare event that a ZvZ game ever lasts long enough for Hive-tech, Hydralisks may be coupled with the Defiler's Dark Swarm and Plague to destroy the enemy Mutalisk flock. However, this tactic can backfire if the opponent has Zerglings, Lurkers or Ultralisks to send under the Dark Swarm to attack the Hydralisks.
Hydralisks are used commonly against Protoss, and can be very powerful against Protoss when massed. The Zerg needs to be good at Storm Dodging to avoid getting all their Hydralisks killed instantly. Hydralisks are often used in a combination with Zerglings, Lurkers, or Mutalisks.
If the Protoss decides to use Sair/Reaver, the Zerg often chooses to make Hydralisks to deter the Corsair harassment. Zergs can also research Burrow and burrow a Hydralisk group in a spot where the Zerg player thinks the Protoss air fleet will fly over. When the Zerg player sees it fly over, the Zerg player will unburrow the Hydralisks and can snipe the Shuttle carrying the Reavers. The Zerg can even use an Overlord as bait to lure Corsairs onto the burrowed location.
Hydralisk masses can easily take out buildings, thus they are sometimes used in drops on Protoss Bases to wipe out key buildings such as the Templar Archives or the Nexus. When paired with Lurkers in a Drop, Hydralisks can take out Observers that the Protoss brings in to detect the burrowed Lurkers.
Hydralisks can be used to snipe key tech units such as Observers and High Templar. In the case that the Zerg has Lurkers, sniping Observers has the effect of increasing the potency of Lurkers. The Zerg can also take advantage of armies with low Dragoon counts and snipe High Templar. Hydralisks are also very effective at sniping stray Zealots or Dragoons.
In the Zerg vs. Protoss late game, Hydralisks generally fall out of usage due to the raw attack power of Zerglings with Adrenal Glands, but if the Protoss decides to pump Corsairs, Hydralisks can be added back into the Zerg army to compensate.
If the Protoss has a +1 ground attack upgrade advantage, Zealots and Dragoons can kill Hydralisks in 5 hits instead of 6. Thus the Zerg should attempt to pursue the ground carapace upgrades for Hydralisks.