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. 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. What I do strongly recommend to do, rather than re-spec your Assassin, is to put it on hold and instead make a Sorceress. Possibly Chaos Sanctuary runs, though this may be a bit slow.Do Countess runs in Act 1 to farm runes, transmute them, and make lower end runewords.Things I suggest you do if you take this route are: 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. One thing you can try to do is to re-spec into a Trap Assassin, primarily using the Lightning sentries to kill monsters. 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.

In general, melee Assassin builds are better served for PvP rather than PvM. Pretty much all builds will struggle in Hell without some decent gear to put on your character.

Unfortunately, Diablo 2 is a very, very item dependent game, which is probably why you are struggling on Hell mode.

Diablo 2 lod trap assassin build 1.13d