@StrikeALight: There is no way that Hydra is harder than the Darkroot Basin one. Then again, I beat it when I was like level 67 or something. I used the same tactics; roll away from projectiles, get closer and when it attacks, chop the heads. At some point, I was hurting it so bad, I killed it without even chopping all of its heads.
It totally is harder though. It's got better range, there's a lot less cover, it can jump to the other side of the lake, and there are a bunch of effing clam things along the way to damage you. The one in Basin is a joke in comparison, that thing never even hit me.