Diablo 2 Walkthrough -> Assassin
Also remember that the Assassin is the only character who doesn't require keys to open locked chests. Here are her three main skill schools:
Martial Arts — this school's skills are divided into two types: charge-ups and finishing moves. Charge-ups act as "batteries" for subsequent finishing moves.
The fight goes something like this: first we build up strength for a strike (with each new charge-up, the power of a specific skill increases significantly), and then we finish off the enemy with a well-aimed finishing move.
By the way, it wouldn't hurt to place them under different mouse buttons—with some skill in using such combinations, you'll be able to take down almost any monster with one hit.
Shadow Disciplines - Most of the skills in this school are almost complete analogues of Paladin "auras" - knocking down an enemy, increasing speed, reducing the duration of poison, etc.
Other skills are based on mastery of a specific weapon type. For example, after level 12, you gain the ability to create a virtually impenetrable block using two "claws" (the Assassin's special weapon). At later levels, Shadow Disciplines allow you to summon your own "shadow" (a complete copy of your character) to provide assistance in the heat of battle.
Traps is a highly controversial school, somewhat at odds with the Assassin philosophy itself. Almost all skills here are based on the ability to set various traps, such as fire mines that explode underfoot or portable launchers that shoot lightning bolts.
However, in the later stages of the game, these skills lose their dubious usefulness, as setting up any trap takes a relatively long time, while the damage they cause to serious enemies is laughably small.
Tips for leveling up
From the very first levels, start leveling up your Tiger Strike and Dragon Talon Martial Arts skills. You can even assign them to your left and right mouse buttons. The former is invaluable—for just 1 mana, your attack power increases by 100% each time, and your Attack Rating by 45 points.
Dragon Talon is a great choice for instantly dispatching Shamans, small bosses, and other undead from the early levels. However, don't invest more than one skill point into these abilities—from level six, you'll gain something more interesting, like Fists of Fire: at level three, it's the best way to clear out numerous clusters of annoying little creatures.
At the same time, start mastering another school - for example, Weapon Block would be very useful (be sure to assign it to a function key - a not very strong Assassin will often have to defend herself from superior enemies) and Claw Mastery at a level of at least 3 (provided, of course, that you decide to play exclusively with claw-class weapons).
As you gain experience and levels with Fists of Fire, gradually switch to Claws of Thunder, which is recommended to be developed to the very end as the best charge-up skill.
In Shadow Disciplines, the Shadow Warrior skill will be absolutely indispensable. Make it a rule to add one to this skill every two levels until you grow to truly stratospheric heights and learn the more advanced shadow assistant - Shadow Master.
Source: diablomania.ru.
See also:
Diablo 2: Druid Skills
Act 5 - Meeting with Baal
Half-Life 2: Dark Energy