HU is intended to be a challenging mod (for games like this, that usually means you need to figure out the details and gear around that), and the realm version is intended to be played with a few players in a team, but you might be able to find gear that fills in some roles the other players would fill.
For a Paladin facing Duriel (I'm guessing the big hurdle is getting enough HP to survive a big hit like that 4k Shockwave), it would be a good idea to get some PhysRes (Shockwave is phys damage), and maybe get some source of the Weaken curse (Weaken reduces phys damage generally, and most phys-dmg spells have been soft-coded to consider this (Duriel's Shockwave happens to be one of them)). In HUv2.1 Weaken reduces damage by 15% (25% if maxed LowerResistance synergy), which would negate about 600-1000 damage from Shockwave. The webpages show that a Skull'd Shield and maybe Ber'd Armor/Hat would be decent sources of PhysRes, although PhysRes seems like a pretty common mod on Uniques (77 sources just looking for "Resist Phys" on the Unique Armor webpage). Lastly, keep an eye out for +%HP or flat +HP mods (Not +HP/Lvl or +Vit (unless they're really high) since they aren't buffed by +%HP), or just trying to level up (probably not a good option after Normal/NM) and raise your Vitality more.
If those three methods of getting over a big physical hit aren't enough, then that does sound a bit unfair. Consider adding a comment to the current
HUv2.1 Feedback thread (maybe include your character build,relevant stats (E.G. PhysRes, HP, Weaken% in this case), team builds, and then request some nerf (be specific, like "-20% Duriel Shockwave Damage")) to whatever is giving you grief.