currently the best thing a hydra can do is be teleported into the enemies spawn point and sit there and attack everything that comes out. or a swarm of them (non-legendary) well placed so people can't avoid then for ever.
I disagree with your comment. My friend likes to play a wizard with a lot of metamagic and control-type spells (full complement of reverse magic, jinx, nullify, as well as teleport, force push, and jet stream). He summons the hydra in the same zone as my mage and then repeatedly teleports me back on top of it.
The hydra is quite punishing with its 3x3 attack, especially against unarmored mages (which I tend to be, because he likes to dissolve armor). It is hard to put up blocking creatures because he will drop essence drain on it. The hydra itself is difficult to kill with its high health, regen, and COUNTERSTRIKE.
The combination is not unbeatable, and the hydra is particularly weak against a mage with lots of strong attack spells or a finite life enchant. But hydras do well against swarms (because of counterstrike and regen) and hit hard if teleported into position.
Though it isn't the best creature by far, I still rate it as an excellent spell because it is one of the best creatures for what it does (hit hard when not required to move).