Welcome to GD. Best aRPG if not a little outdated technically.
Anyways, comparing items, don’t look at the dps stat beyond early levels. It only accounts for a fraction of true dps. Get Grim Internals (resources tab on this forum) it has an overlay that shows dps of different dmg types accounting for procs, crits, buffs, debuffs, wps, etc.
Or just go by feel.
That must be Zantarin, undead nemesis. Another reason to get Internals. You get hovering enemy names. Then you go grimtools.com and check them out. There, you’d see he has a killer shotgun skill with million projectiles, hard to survive without flat dmg absorption (like Ascension or Inquisitor Seal) and better to wait out.
Truly, this game is pretty much impossible to play beyond a certain (quite low even) level without research.
Also, I checked that build followed and it looks random. It’s definitely not optimized. Unmaxed ABB on full Venomblade? If one want pure acid melee on Fervor there are better things than full Venomblade. It won’t make a big difference at this point for you but if you wanna learn the game better follow competent builds. Check out @grey-maybe ‘s list GrimSheet: Endgame Build Table - ETE edition [v1.41] - #435 by A_S00 for (IMO) some of the best builds by some the most accomplished guys in this game.