No weakness, although some people believe him to be weaker to Magic than to other infusions, and some believe he is weaker to strike damage than other physical damage types.
Melee: Roll through his attack(s), hit him on the butt, repeat.
I am in Ng+4 now and my roaring halberd (raw infused, 2H, 1H is too slow) with it´s dark damage hits him from 300 to 600 (i guess on his head) so maybe he is a bit vulnerable against dark damage
More seriously, upgraded Hexer's set, a good 2H weapon, max stamina regen, load under 30% and Steelheart Ellie as a phantom, make this fight a lot easier. Got him on my third try with this setup.
Hypothetical Sunbro : "So, is this boss fast or slow ?" Me : "Both." HS : "Does he do combo or heavy attacks ?" Me : "Both." HS : "Is he immune to bleed or to poison?" Me : "Both." HS: "Does he do physical or elemental damage ?" Me : "Both." HS : "Does he do a 3-hit or a 4-hit combo ?" Me: "Both." HS: "Does he telegraph his attacks or is he unpredictable ? " Me: "Both". HS : "After finishing his combo does he jump away or does chain another attack ?" Me : "Both." HS : "Does he give you time to heal or does he attack you too quickly ?" Me : "Both." HS : "Does his attacks reach you behind his back or is it a safe zone ?" Me : "Both." ... HS : "Is it a fun boss or a frustrating boss ?" Me: "Both !"
Remember when this boss was infamous for his high damage delay sttacks that wait until they think you will roll? Now every boss in elden ring does it 3 times over. Ahh, simpler times.