Im currently on NG+ and this guy hits really hard. The easy strategy I found was using a upgraded tower shield (not max) since it blocks almost 100% of his shots and atacks. The opening is when he shoots to the air and when he jump atacks above you since he will be standing with his backs to you. Take one hit at a time and always have your shield raised, it consumes almost no stamina blocking.In phase 2 try heal/buff weapon instead of taking one atack when you have a opening. This boss really needs patience since 1/2 shots of his pistol in quick sucession will most likely kill you.