

Decrepify will also work, though I'm not sure if that actually comes as charges on a wand. 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). 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.

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.

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.
