Hi
Number 1 drop the air magic, you want aoe spells that affect lots of hydras at once, you also want to avouid having to split attribs between several ddifferent lines.
Number 2, max out earth magic, the extra armour reduces damage significantly.
NUmber 3, if you are having trouble with knockdowsns, switch to warrior and take balanced stance
Number 4 remermber that although dispatching enemies quickly is good, taking longer to kill one, yet being able to surviv e larger groups is normally a better payoff (even if its just for the fun of seeing 1 spell do over 2000 damage (easily done with crystal wave against hydras))
Number 5, you need some sort of energy management skill in there, mty favourite for geotank is ether renewal, as you should be using at least three enchantments on it anyway.
So after those here is my hydra build:
stone daggers

cheap spammable, mainly to keep kinetic up but can also be handy for damage if your only takign on one enemie for whatever reason)
aura of restoration (if your spamming stone daggers, dont underestimate the healing effect of this skill, plus it helps with energy management in this build))
earth attunement(makes skills cheaper to use, and is especially useful cos of the energy management skill)
armour of earth (well, this is one of the two core skills of any geo-tank build)
kinetic armour (the other core of any geo-tank)
ether renewal (voted most useless skill by many people, but is INCREDIBLY useful in geotank builds, should get it so you refill your energy bar form 0 in its duration)
crystal wave (the basic damage skill of this build, not sure whether aftershock would work better due to energy/recharge thoguh)
**balanced stance** (for hydra farming this is what ill use, but against non kd enemies other skills can be more useful, maybe mantra of resolve, or just another damage skill)
basic tactic is grab as many hydras as you can (im pretty sure ive had 20 at once) take their hits easily and use ether renewal whenever its up for regaining the energy, plus heling any excess damage, (but make sure balanced stance is up for this otherwise you might end up with 0 energy for 30 secs, and thats no t easy to recover from).
oljomo