How to easily defeat the gaping dragon in the depths
You will need a overhead slash weapon (like a halberd or axe or great axe and whatever armour you want. A shield that blocks 100% phy damage and lots of stamina
When you enter the arena don't charge it let it come to you but try to keep it away from the walls and wait for it to charge then when it's recovering hit it's tail with the overhead slash (running one-handed light attack for halberd heavy one-handed standing attack for axe and great axe) attack around a faint ring at the base of it's tail for a quick sever after tail is severed wait for it to charge then attack it. Then repeat to kill but always be cautious. NEVER stand in front of it if you are close to it. Try to keep it away from the walls but you don't have too and don't get boxed in as almost all it's attacks are one hit kills. Don't risk an extra attack only hit when you know you wont be hit.
More tricks, tips and cheats for this game are right here - Dark Souls cheats