make sure your tank isn't putting his back to any mob, they will crit you to death. focus fire on the casters first. Arthas can take a few hits, especially on the 3d pack, don't worry about pulling stuff off him right away until you have solid aggro on the others. Don't loot anything upstairs when you first run this instance, most of the dragons don't even drop anything and you don't really have time to gather everyone up and run to the next spot and loot, just focus on moving.

The hardest position wise is the 3d one in the hallway, since 2 come from front and 2 from back. You should have to gather them up best as you can with your tank (let arthas take 1-2 and taunt off him after 10 secs is easiest), be ready to have your team follow your tank around and reposition. Its very easy to die on these if you aren't quick on heals.