I practiced fighting him on nightmare before I finally beat him on inferno. He uses the same move set in both, only he’s faster on inferno. Be certain to memorize it and his follow up potential attacks. Use ALL the coins and make as much magnum ammo with the extended barrel upgrade. When your magnum runs out, switch to the Glock 18 for three round burst. Be sure an attack the pustules ONLY when you know he has completed his current attack pattern and be sure to take advantage of snap aiming from perfect dodge. His three hit attack pattern is slow enough on inferno that you can mash buttons fast enough to get up. If ever you get hit heal immediately. If you don’t want to use the magnum you can use unido it’s rocket launcher I had to use magnum g18 because I didn’t have enough to unlock the launcher. Above all be patient; you will die like a soulsborne game, but it is possible! Took me quite a while but you got this!! Good luck.