Damage - Hydra does this pretty well, hitting lots of targets and inflicting poison
Leader - He is the core of any damage over time team
Dragon - Can be used in deathmatch and keep away from Dragon Slayer
Tasty - Not sure how poisonous snakes are tasty, but either way keep Spider away
Reverent - Can be revived by Angel Dragon
Magical Damage, physical skills
Viper's Nest - Not a bad white skill. This has snakes bite the first 3 foes in the lineup for physical damage. This is mostly nice because it is always 3 hits, so if there are less than 3 targets it will hit the same one more than once. However, the damage of each individual snake is not that strong.
Deadly Poison This is where things start to get good. This is a passive skill that poisons any foe hit by any of Hydra's abilities. So pretty much the front heroes are gonna be poisoned the whole time. The damage isn't that high on it's own, but it can easily be boosted. The poison lasts for 4 seconds and is re applied with each hit.
Sidewinder - This is a decent skill, but mostly just a vehicle to spread poison to more targets. Sidewinder makes Hydra's basic attack follow the same pattern as Druid's hedgehog and bounce through the first 4 targets. Keep in mind that the bounce has a small range, so it won't jump that far to try to get to another target.
Cobra's Dance - Interesting passive skill. This enhances Hydra's Fury by a certain amount. Fury is an interesting stat. Basically, how it works is that once your hero gets hit for damage past a certain amount of their health (10%), they get energy to fire off their white skill. This passive boosts the amount of energy you get. This makes it VERY easy to fire off Hydra's Vipers.
Sharpened Fangs - And here is the main reason that you will be using Hydra. This skill boosts the power of damage over time skills by a significant amount. This applies to your own skills as well, so Deadly Poison gets a huge boost just from one level in this. This skill is a great damage boost, especially since it's a percentage boost so it scales excellently.
Oh boy, runes are....tricky on Hydra. There's a lot of viable ways you can go, since the stats for his attacks are kind of all over the place. Also attack speed increases help spread poison faster, so bolt is also a good idea. I wouldn't necessarily recommend Tree on him since lower HP makes it easier for Fury to trigger, and as long as Fury triggers energy gain isn't really a big deal for him. Beyond that....have fun! I personally went with Rock and Flame runes, but that's just me.
Plague Mogul - Do it. Do it now. These two were made for each other. Hydra boosts the damage over time, Mogul boosts toxic damage and they stack with each other. These two are the core of any damage over time team.
Dark Dracul - A great pairing for Hydra. They're both magical/physical mixes but their DoTs are the opposite type so they stack very well with each other.
Ancient Idol - A great control unit and he has 2 DoT skills to boost with sharpened fangs. A great choice of a pairing.
Bone Dragon - Tar Pit is DoT and Bone's energy drain is always useful, but I personally have never had this pairing work all that well. Your mileage may vary.
Ambush Spider - Great for taking out a lot of annoying targets, and Lethal Weapon is boosted by Sharpened Fangs. Spider can also function as a tank in some situations.
King Kaiju - Hellfire is a DoT and like Spider he's good for taking out some specifically annoying targets.
Cyclops Shaman - His white is a DoT and his recharge helps out his whole team, a good pairing.
Sun Seeker - Probably the most consistent option for a tank on a DoT team, with two skills boosted by sharpened fangs, more bulk than Snap and not as dependent on the other team having tasty heroes as Spider.
Dragon Slayer - I don't think I have to explain this one
Ambush Spider - Not only is Hydra tasty, but spider is able to jump over any tanks and take him out instantly.
Silent Spirit -His wall stops Hydra's skills from hitting multiple targets, and Tea Time restores HP faster than his poison can take it away.
Unicorgi - Hydra tends to sit pretty close to the front, and corgi's tail spin can usually hit him.
Void Wyvern - Void's white clears away any poison you build up, and his energy drain will make it harder for you to get started.
Vile Bile - Vile will focus all your attacks on him, and then explode to punish you for all the damage you dealt.
(+) Sharpened Fangs is a fantastic skill that boosts the damage of several skills
(+) Hydra is great at hitting multiple targets, spreading his poison all over the place
(+) His poison is a strong damage over time effect and is easy to boost
(+) His high fury makes energy gain pretty easy
(-) His base damage is fairly weak
(-) He has terrible defenses