Hit an run, thats what your supposed to do, well you can do anything really, but it's what I do.
I jump in, either dashing or warping. Then I spread some damage and bleeding (maybe poison) then jump back out with something like return or heart of shadows. Heal, rinse and repeat, or you could go into the fray and spread round some blind (with blind powder) so that at least the nasty meele npc's can't hit you. You could also make a squishy (spell caster) killer, warp to the spell casters and deal as much damage as possible then warp away before they know whats hit them. Thats what i at least think assasins are for, run and hit, or to focus on one type of character.
Some of my fave skills are things like shadow of haste, run in attack, then after 30 or so seconds, your away from the fray to lick any wounds.
Put up that critical mastery, take advantage of the assasins chance to double hit with criticals, or use a sword to do major damage with critical mastery. You could also try combining barrage with sharpern daggers and critical eye with a bow,think of the spread bleeding as well as critical hits. The combinations are many.
I love assasins, but i don't think they're the hardest to play as, they just need a bit more thinking than 'tank tank tank' or 'heal heal heal' not that i'm saying assains can't spam skills with the best of them :P.
Some viable secondaries would be:
Ranger: barrage + critical damage from assasins = lotsa spread area damage, also you can use the ranger stances to great effect.
warrior: frenzy is aparently a very viable source of fast hitting power for assasins. Also combing swords and axes with critical attributes is nice.
Monk: well, i suppose it'd be the same for most classes, try and take advantage of vigorous spirit since you'll be attacking fast and hard.
Ele: well there are a lot of sliver armour solo builds out there for assasins, go check them out.
necro: status spreading, need i say more? warp spreading conditions can be pretty nifty.
mesmer: well the usual mesmery things like illusionary weapon, ect
rit: well.. ehm.. can't think of anything but i'm sure theres something out there :x
Remember people these are only my opinions.. and it's based on pve assasins, not pvp.