The 6th boss you will fight & the first lord of cinder is the abyss watcher (easily one of dark souls best bosses).
In the arena there will be other enemies that will not only attack you but also attack the boss if you're lucky which gives you a chance to attack him while he is distracted. As a melee character I simply used a shield and moved around him and dodged him when needed and got some back stand on him but be warned he does move fast.
Although he may seem easy at first, once his health bar drops the second phase of this fight kicks in and he will become the lord of cinder form. The fight is basically the same but his attacks have more of a range and damage is increased but not by much.
Below is a video from powerpyx: