Lots of people used joharaoc.eu for their feats, but the website looks to be down. So here's a link to the build I use from aocfeats.com
http://aocfeats.com/feat/hox/12b1adb...bf441be1a1be7c
This probably isn't an optimal build, as I use this as a personally-tailored build that does decently in both 6-mans and solo dungeons so it suits my purposes for the most part. If you want, you can take the feats in the Possession tree out of Roots of Hell and Improved Word of Command to put into Spitfire, I personally like the extra range cc and cc resist for tackling mobs in the field.
For the most part, it's a solid Avatar/Poss hybrid build. In the Avatar tree, BtD buffs are a must, PoIF cooldown reduction is handy, can then slot two PoIF's (II and III) into your combo rotation and, alongside Fire Lance and BtD, you will have pretty much no downtime between combos and spells. Insta-Inferno Curse becomes a viable debuff for the HoX.
On the Poss side, using Inferno at max stacks is really great for laying down the AoE and helps when you need to be on the run. Off the top of my head, I can name Runemaster Narvi in HoC and Caverns of Malice HM as two 6-man instances where it comes especially in handy for taking on lots of mobs. Phoenix Wings makes Phoenix Cloak bearable and handy as an oh-**** button in tough spots where heavy-hitting mobs turn on you, as with Zealots in the High Priest Syth-los in HoC or again, the mobs in Caverns of Malice HM, especially when your group is clueless or otherwise preoccupied and can't yank them off you right away. The one point in Molten Flesh is just for niche survivability and showing off. Ignore all the fire attacks from bosses with it, great for pulling away bombs from the Challenger in the 5 bosses in HoC.
Try it out, tweak it to your own liking, and have fun.