Umm... You really didnt get the idea, didnt you? Lemme explain it to you. When the Hydra (Or Ice Hydra) is attacked, it gains +2 ATK BECAUSE of the tale that it regenerates 2 heads when you slice 1 head off from it. Of cource, you cant do it that when its attacked it gains +2/+2 because that would make it almost immortal exept from fire lances. Of cource you could kill it with more than 1 spell, but if you dont kill it with 1 shot it becomes more powerful. Also its very resistant to poison because it gains +2/+0 from each damage dealt, and if its poisoned by 1 poison damage, it gains +2 ATK every turns. For example, the un-upgraded Hydra lasts for 6 turns so its 6X2 to ATK wich is 12. And there you already have dealt 49 damage (Dunno if i calculated it right) within 6 turns. I dont know if this cleared up anything but still, try to understand the idea, will you?