the assassin is a bit of an oddity, and has a large learning curve. here are some of the things i learned the hardway, but made all the diference in playing the class successfully.
first off - the max dmg for daggers are 17 B4 armor, which equates to next to nothng after...... needless to say if you are useing auto swings anywhere in your fighting you're going to be loseing more dmg then your dishing out, by ten fold. - so you only want to be in mele range long enough to throw your spike.
secondly - the larger your combo, the more you're going to get hurt... i stay with a 3 attack combo, but many have success with 4 - i would not suggest any more then that, as not only do you spend more time getting pounded, but you are loseing valuible skill slots that you need to use for avoiding dmg.
thirdly - the assassin works best if they are the last one to enguage in battle.....if you rush ahead of your war (or tp ahead of him), you are going to be earmarked as target #1; if, however, you wait out of range untill your war has drawn his agro, and some of your casters has drawn the rest - then you can run in and whoever you attack are already going to be throwing down with someone else and won't get their agro switched you faster then your spike.
------the best overall group tatic (that i've found) is for you to stay back and let your group start the fight, then if an enemy war runs after your monk, or other casters, you take him out first - then after your casters are safe, you then start going after their casters.
------if your useing henchmen, you can still use this tatic; just use the shift- ctrl- space command. - ctrl+space calls and attack's your target, adding shift lets you call the target without attacking. this way your henchies will attack who you call and you can stay back untill they have all the agro.
forthly - Avoid dmg ( this should actually be your #1 priority, but without knowing the ones b4 it doesn't hold the same weight).... only be attacking long enough to throw your spike, when it's rechargeing, or you're regening energy hang back by your casters, or use the dmg avoidence skills. - if you have a secondary that has ranged attacks (bows, wands, sceptors) you can bring one in a secondary slot and keep knocking out dmg if you decide to hang out with your casters.
fifthly - bring condition removal.... alot of the inefectiveness you see when you're first learning is comeing from conditions. - blind is amoung my least fav, cuz you end up sitting right in the line of fire, literly doing nothing. but bleading, deap wound, posion and what not are all pretty nasty too with the low ammount of health you have.-- and hard to notice as you're trying to focus on getting your combo off right and getting ready to bail out.
and lastley - unless you're bringing a 5+ hit combo ( which will most likely get you killed) you're not going to drop your target with 1 trip through your spike.. either deal a big spike and let your casters finish em off - or bring a nice little range skill from your secondary to finish em off yourself, or run back in and tag him again as soon as you can.
my basic set-up/ stragedy is like this
3 attack combo
shadow of haste
condition removal
shadows refuge
range spell finisher
res sig
note: set your shadow arts attrib to a number that will time shadow of haste with the time it takes you to use your combo... if you max it and have a 9 second combo, then you end up spending 8 seconds in danger that you can't afford.
and the basic stragedy is to hang back and take out any rushers, then recharge everything and pick one of their closer casters...use shadow of haste while you're runing in, and throw your combo as soon as your in range. if you set shadow arts right you'll be teleported back by your casters as soon as your combo is finished, and just b4 your target gets to swing at you (cuz he'll have to switch his targeting from whoever he was shooting over to you; the AI won't even start that untill you've hit em). then fire off your finisher spell.... refuge will be more then enough to heal any miniscule amt of dmg you take while in there, so throw that; then pick a new target and wait a bit till you can repeat.
this is a standard shadow of haste build/ stragedy, there are many different ways you can go depending on if you use AoD, recall, or any of the random tp's to keep stacking up conditions - but it's the spike and avoid dmg concept (hit & run) that you have to take to heart. - hope it all helps, GL & HF