First and foremost, this dragon is incredibly stupid. This is, what he does: when you stand in front of him, but way out of reach of his slam attack, say, about twice of his reach, he STILL TRIES TO SLAM YOU, like you're standing right next to him! So, obviously, he totally fails which makes him vulnerable. So you do a few quick strikes, go out of his way, because he will dash, and repeat the whole process. Remember to always, ALWAYS be out of the reach of any of his attacks and I can assure you, he will be no challenge at all and no flask will be spilled.
For this particular boss every other strategy is inferior.