Guild Wars Factions: Assassins in PvP
In PvE, Assassins are having a rough time getting into groups (a) because there's a ton of them and (b) because a lot of people that are playing them aren't very good ("omg i play the ninja!!1!"). The key point is that you're effectively naked compared with a warrior and so just running up to a bunch of monsters and standing there hitting them isn't a great survival tactic. Imagine if you had Frenzy up the whole time as a Warrior and you'll get the idea. Hit and run is the key, just as in PvP - take out their most irritating caster, teleport out, recharge, rinse and repeat.