When most people come into the game, the first type of unit that really catches their eye is the wyrms. Unfortunately, most of these units(especially the dracolich) become less and less impressive as you gain more experience.
The Hydra is not one of those units.
Strong against: Almost anything/
Weak against: Mimics(like all expensive units), artificer + changling
Best Paired with: Sergeant is a must; mourners, confessors, strategists, changling(against AI)
The Hydra is one of the few very expensive(costing over 400) units that doesn't cause an experienced player to throw their hands into the air in frustration when drawn in random; even without its soulmate, the sergeant, it's a tank complete with serpent turrets smashing its way up the field. Of course, it is very command intensive without a sergeant; it costs five commands just to deploy all snakes from a full health Hydra.
Now, pair it with a sergeant and it becomes a nearly unstoppable force, to the extent that there's typically something of a "gentlemen's agreement" in constructed and 2v2 that neither player bring a sergeant/hydra setup.
Of course, most players do not have such an agreement with the AI. When playing single player, my setup is always the following(I almost always win in less than 5 minutes):
Changling x 2
Gate Guard or Acolyte
Deployment order: Sergeant/Changling --> Hydra --> Strategist(Changling can precede the strategist if your Hydra's snakes aren't going to get attacked that turn)
The Hydra can destroy almost anything within range; its snakes block the AI's units' path and, most importantly, provide fodder for changling swaps. The changling is used to swap for the AI's best units; typically, the AI will then proceed to trade the changling for a serpent, letting you repeat the process over and over, grabbing valuable units in exchange for serpents. In any round, the AI will destroy many of your serpents, feeding commands to your strategist, allowing you to move all of those serpents and stolen units.
I would typically advise using the 'new unit' button when you're new to the game and doesn't have almost all of the units; however, it might be worth the money to directly buy a sergeant, a changling, a Hydra, and a strategist: this setup makes single player a cash cow.
Some warnings: Be very, very careful about heretics and warriors- heretics and warriors(or something that can be used to 'de-activate' them before they reach your snakes) should be changling swap priorities. I've occasionally carelessly let an ascendant warrior cleave through my snakes all the way through to my Hydra.