Here is a link you might find interesting Card Design (
http://elementscommunity.org/forum/index.php/topic,16655.0.html)
Your Hydra is balanced. (See spoilers for walkthrough on why I say that)
However why would a Hydra be a Water creature? Hercules fought the Hydra on land.
Starting with one of the heads
Unupped
5 [5Attack]
0 [0<HP<6]
-1? ["Hydra Head is destroyed if you do not control a Hydra"]
Cost=4
Upgraded
6 [5Attack]
0 [0<HP<6]
-1? ["Elite Hydra Head is destroyed if you do not control a Elite Hydra"]
-1 [Upgraded]
Cost=4
Looks good.
Note: The value of Hydra Head is 4 but the value of Elite Hydra Head is 5 (the cost reduction due to being upgraded doesn't affect the value calculation)On to the body
Unupped
1 [1Attack]
3 [9<HP]
6 [Spawn a creature worth 4]
-1 [Skill costs 2-3 quanta]
=9
Upgraded
1 [1Attack]
3 [9<HP]
7 [Spawn a creature worth 5]
-1 [Skill costs 2-3 quanta]
-1 [Upgrade]
=9
Again it looks good.