Run MfD on little adds and Rupture everything. Kick everything. Use Cloak a lot. I also specced Elusiveness and ran Crip; there's a lot of stuff you can Feint on this fight and Crip is helpful. I was also in charge of channeling orbs since I was in melee range anyways. Use CDs on Horridon between gates.
For the final phase when you will get a Direhorn Spirit fixate, make sure you are at max melee range (the dinos spawn in the middle of the boss and can immediately 1shot you). I used this macro to knock it back:
You can knock it back with FoK too, but I found that was a bigger DPS loss than losing a second of uptime with Throw.
/tar Direhorn Spirit
I ran the Smoke Bomb glyph too and just used it as a tank CD.
I died a lot on our early attempts, but once you get more experience with the fight, surviving shouldn't be an issue. On the 4th door, I spent a lot of time on Horridon instead of the adds because the cleave and totems and curse and flamecasters were just too much to get any real uptime once everything was out. Damage on Horridon is always helpful as long as the adds die in time anyways.