Common Strategy:
The instance that contains Harla Dar is the same as SoS / Temple, but is only the section with the gorg pit / platform (where Harla Sits).
There are tons of trash mobs including drakotas, lizards, and droags. HD continuously calls for aid during the fight, which will bring any trash mob thats alive.
The mobs are on a 25 minute respawn timer, so you need to kill all the trash as fast as possible to give you enough time to kill Harla Dar.
One way to clear the trash quicker is to just /yell everything. Also to start, corral as many mobs as you can together, get all their HP as low as possible - note drakotas regen fast - and then blast through the first group, killing many in quick succession.
From there just work your way around slaughtering everything.
I would definitely suggest as others have said, to bring self-cure potions to make up for any slack in group healers.
More information is needed.