Moonfire it enough times to keep it away from you. This can be a bit problematic on the last phase. Make a /tar macro so you don't have to click it to cast MF.
Use sprint, Stampeding Roar or Displacer Beast to get as far away so you can heal.
Tanks on Horridon simply need to move out of things that are avoidable. DPS should never get killed by the dire. People need to be vocal when it comes out and who its on.
The most difficult part of this encounter is interrupting the adds and killing them. Don't stand in frnt of warbears. Make sure to be on you A game with the dispels. Pick a group for poison and have the pally do the other. Always dispel the highest stack person first. It should never get over 5. Last door it's on you do make sure you dispel on every CD. After that it's the healing on the last phase. If you need to spam RG do it but only on the tank on Horridon. WM are kinda iffy here imo unless you ranged has a set place to stack. Most likely it won't go down like that with all that's going on in that phase. Just have tranq up for the first CD and use Incarnation as another. Make sure to use Ironbark on the horridon tank there too he is going to be hit like a truck.