If you have decent enough vitality and endurance, the best tactic against this boss is to forget any tactic and go full melee berserk. Equip the toughest piece of armor your have, preferably havel's with havel's ring (your movement speed doesn't matter just keep load under 100%), get highest *****ing dps weapon and highest stability shield you have in inventory. Go to the arena and start running towards boss with shield up. Go as close as you can, apply resin to your weapon, and start whacking that *****er. Watch out only for explosions or grabs. Your high poise from armour will let you to chug estuses without getting stunned. He should get down pretty quickly. This "tactic" worked for me both in NG and in NG+, killed him on my first try in NG+.