Board Thread:Dark Souls/@comment-9954580-20130607043137/@comment-1852943-20130617123250

The leading cause of death when fighting the Hydra is falling off the underwater ledge. You want to stay close to the edge to provoke the Hydra into attacking with its heads and not its magic, just keep a good eye on your footing. Do not lock on to the Hydra. Use your shield to block it's attack (most of them will miss you), and all of its heads will get stuck in the ground. Turn and attack the nearest head before it retreats. After a few hits, you will sever the first head, and its attacks will get weaker. Keep up this pattern and the Hydra gets weaker after each head you chop off. When it's down to only one or two heads, you may want to run back and forth to get to the head before it retreats, but that is a good way to fall off the ledge and die. Just be patient, and remember the more progress you make the easier it gets.

Alternately, if you have a ranged attack, it may be easier for you to shoot the heads when they're stuck in the ground rather than run towards them.