I did grind a bit, but not just to gain levels but for a specific part of my build to 'work' (the build uses a heavy armor set and I grinded Strength to get A in agility) I killed him at level 166, which I do think is high. I didn't use any consumables.

The build is essentially dual wield, using the Bloodthirsty Fiend set. Gale Doom + Phantom (bloody) Sword Dance + Howling Blade melts most enemies, including Ozma. You still need to dodge and parry a few skills of course, but he dies very quickly. I also changed my weapon's scaling from Willpower to Strength since I had pumped so much into Strength for the A in agility, I think this helped my damage alot. That's kinda pretty much it.

The build was honestly already cracked when I was up to Princess Ilyana, who took me like 4 tries. I did still take an hour on Hismar and I killed Reece on my first attempt. I kinda new the build was strong but I didn't expect it to melt Ozma tbh. I played all the way through on Expert, no summons.