Gotta face him in the cave, you will see an island of boulder which you can run in circles to avoid his gaitling gun. Try not to go to the "inner" inner section of the cave before taking the hunter down. Otherwise you will trigger some other creatures to join in the fight.

This tactic saves me from the frustration. If you carry less than half of the available blood vials during second phase just restart. It's pointless to continue on. Spamming fast firearm/blacksky eye can score you a visceral attack at the beginning of the fight. Proven working on my NG+4 so not sure how ng+7 gonna be.