My guild recently downed 25 Jaraxxus with eight ranged DPS, six healers, three tanks and assorted melee.
Jaraxxus was tanked in the middle of the room by the MT, melee stood behind him.
Our raid healers stood at the four cardinal points of the star in the room (corresponding to the three entrance gates and the large gate where the beasts enter).
Our tank healer stood near the tank.
Our OT healer was mobile (following the OTs).
We split our DPS into four groups of two who stacked at the smaller points of the star. They kept up DBMs range check at all times (15 yards). This setup kept chain lightning from ever hitting more than two people.
Our elem and enh shams purged constantly, and our mages spellstole Nether Power as often as possible.
During the fight, our OT picked up the mistresses and had the nearest two ranged DPS groups burn them down.
Then our second OT would pick up the infernals as they spawned and hold them near one or two ranged groups. If at any point the infernals charged the melee (when they turn into a little ball and fly at someone) our melee would peel off and DPS the other infernals down.
Anyone with Legion Fire would run to the outer wall as fast as possible (this was challenging for the melee) and we always had at least two healers in range for Incinerate Flesh (which often targeted a healer themselves).
We downed on our first attempt after implementing this strategy though it was sloppy. We probably spent about 45 minutes overall on this boss. My guild is not a hardcore raiding guild.
The most important aspect of this fight seemed to be avoiding legion fire and the infernal's AoE (I can't recall the name at this moment) as well as interrupting Jaraxxus' firebolts and keeping him without any nether power. We were unable to determine the actual distance that his chain lightning would jump as some people reported being hit even with nobody showing on their DBM range meter (at fifteen yards). Good luck to you all and I hope it goes easily for you too.

