I can't beat the Abyss Watchers

This is the first time I'm playing a Souls game and I'm playing as mercenary and leveling vigor, dex and endurance and I can't defeat them. I tried to run around and let them fight each other, but they always catch me and cook me. I tried backstabbing them but for some reason I cam never backstab them. I tried parrying with a shield and so far that worked the best for me and I got to the second phase but then I didn't know the second phase movements and died. Now I can't pass the first phase again. What do I do to git good?