I know all about getting in groups for mission, the last mission before you fight shiro is the hardest to get in a group.
I think it's not the build that is importent in pve is just how you play, when to attack and when to run away(when 1 or more start to attack you is time to get out fast and let them focus on tanks then strike again) this is how i keep my assasin alive and finish missions with pugs.
for skills:
Temple Strike (elite)
Entangling Asp (i like this 1 if you just could hit the target once and it runs away you can still get to it by casting the spell and it poisons and intrupts the action by knock down, what more do you want
)
Don't take to much intrupts, dmg is much better and you must most of the time get close enough to intrupt as an assasin, so keep enough skills for dmg, healin and defense, the less healing you need the happyer the monks are
