You want to bait his flying attack and roll unset him as he is over you. Block his tail slam and attack his tail. Do a 2 handed attack. You want to stay away from him to make him fly.
There are 2 other times you can get hits in his tail. He will sit up on his hind legs and either breath Black Flame or do his telekinesis attack. You want to roll through his attack and go for the tail.
Equip the DWGR. Remember, patience is key for this tail. --- Rule 2, ignore rule 1. gt pyrotheos