Very simple to use and guranteed to work every time, I will break it down step by step so there is no confusion.
Classes NEEDED: Guard, Minstrel
Recommended: Guard, Minstrel, Champ, two Hunters
1) Enter instance, run STRAIGHT forward and drop over edge
2) Kill the three trash mob spiders.
3) Have all of the party members line up so there are in the MIDDLE of the doorway leading to bosses lair. When you unlock the door have all part members stand in the middle of the threshold. IF you are unsure where to stand then just position yourself against the edge of the door and carefully strafe left or right so you are in the middle.
4) Guard will run to aggro all the spiders and the boss mob spider. Yes your Guardian can survive this..at least any Guardian worth their salt. Any additional buffs you can give them at the outset is gravy.
5) When Guard pulls aggro door will close and party members will be INSIDE the door..it is VERY IMPORTANT that NOBODY in the foor moves at this point.
6) Guard pulls aggro back to the door group who kick the living snot out of the spider adds to get their aggro off the tank. Hunters with rain of arrows/thorns work well for this.
7) Kill all spider adds. Tank is tanking boss..Minstrel is healing tank.

When adds are down, focus on boss. It is important for the tank to keep the spider turned and slightly away from the door group or the acid she drops will kill everyone in the door.
9) Kill spider boss.
10) Loot chest
11) Rinse and repeat.
this will work on almost all doors of almost all of the instances. I have done it for Fil Gashan, 16th Hall, Skumfil, and Dark Delving.