Unfortunately, Diablo 2 is a very, very item dependent game, which is probably why you are struggling on Hell mode. Pretty much all builds will struggle in Hell without some decent gear to put on your character.
In general, melee Assassin builds are better served for PvP rather than PvM. You could probably do some sort of fun PvM build by making a bear sin (Beast runeword weapon with Pride helm and/or shield), though those pieces of equipment are quite expensive.
One thing you can try to do is to re-spec into a Trap Assassin, primarily using the Lightning sentries to kill monsters. Although there's a large amount of monsters with lightning immunity in Hell, making it difficult to get very far without "super great items", this will help you easily do magic find runs on Nightmare mode until you get better gear.
Things I suggest you do if you take this route are:
- Do Countess runs in Act 1 to farm runes, transmute them, and make lower end runewords
- Andariel runs
- Mephisto runs
- Possibly Chaos Sanctuary runs, though this may be a bit slow
What I do strongly recommend to do, rather than re-spec your Assassin, is to put it on hold and instead make a Sorceress. Sorceresses do not require a lot of great gear to survive Hell mode, and in turn make it much easier to magic find and get better items which can help your Assassin.
Now if you REALLY want to stick with your Melee Assassin, you can also break physical immunities relatively easily if the other monsters are giving you no trouble. Try to buy a wand with amplify damage charges from Akara or any of the other wand NPC's and put that on your switch. Amplify damage will let you break PI's in most cases (this won't work on monsters that are normally physical immune across all difficulties, such as Ghosts, or Stone Skin champions). Decrepify will also work, though I'm not sure if that actually comes as charges on a wand.
You also have a few cheap weapon options which might help at first (I'm thinking Act 1-2 here) due to their added elemental damage and other nice melee bonuses, but aren't long term solutions:
- Balanar's Star
- Lawbringer runeword (Amn + Lem + Ko)
You could also hire a Barbarian mercenary and find a Vile Husk Tusk Sword (which has a chance of casting amp damage on strike) for him, but meh.
Another thing you could try is to re-spec into a build that relies on crushing blow to kill monsters, as well as poison damage:
- 20 Claw Mastery
- 20 Venom
- 20 Phoenix Strike
- 10 Weapon Block
- 7 Fade
- 1 Cloak of Shadows
- 1 Burst of Speed
I know you're only level 74, but this may give you a better idea of the end build you should be shooting for.
- Strength: As much as you need to wear all your equipment. Low 100's should be good.
- Dexterity: Same as strength.
- Vitality: Every other point should go into here.
- 2x Strength Runeword (Amn + Tir) in claws (Greater Talons or Runic Talons)
- Goblin's Toe Boots
- Death Set (Leather Gloves and Sash, don't use the Sword)
- Smoke Runeword armor (Nef + Lum)
- Manald Heal rings
Probably the most useful mercenaries for you would be one of the Nightmare Act 2 mercenaries. Either the Might aura one or the Defiance aura one would be best. If you're having trouble hitting monsters in Hell, the Blessed Aim one in Normal Act 2 is also an option.
This build was lifted from here.