Time to discuss the silly Eredar. Not sure why Fizzlebang thought it would be a good idea to summon him into the arena, or why LJ is able to oneshot the warlock with 100k health, but can't kill our 40k tank, but anyway, he's there, so let's kill him!
Pull: usually he's tanked right in the middle. Stand just outside the circle part at the start. If you stand closer, you may get agro even if the tank has consecrate or death and decay etc. up.... and you'll be oneshot (our enh shammy did just that last night on a heroic 10 man attempt). If I use my army on this fight, it's right at pull. Ask for a 6-count before the pull, and summon your army then. Your army can taunt adds, so it's not a good idea at other times, not to mention that they'll tend to die early at other points in the fight.
Icy Touch while running in. Plague Strike. Regular dps rotation blah blah. A tip I neglected last time is if you need to refresh diseases with pestilence but you don't have a blood rune, make sure your blood tap ability is handy and use it! I used to macro blood tap to bone shield, but keeping diseases up is a much higher priority. Again, when ap procs stack, roll diseases and/or pop cooldowns. On regular difficulty, we heroism/bloodlust 8 seconds into the pull. On heroic, we save it for the second Mistress portal since if anyone is dead we'll need help getting the portal down in time.
Be careful throughout the fight. The biggest risk of a wipe is Legion Flame on the melee if the person with the fire doesn't move out and/or the other melee don't get off of it. You may also need to move out of Fel Inferno.
Adds: there are many ways to prioritize adds. Melee may be asked to stay on LJ, but more likely they'll be asked to help kill the Mistresses, the Infernals, or both. If specced Glyph of Disease (GOD), when the adds come up, pestilence to spread your big disease stack to them. When killing the adds, you can pestilence to spread the diseases back to LJ (side note: this works on many fights, you can spread your diseases back and forth. It doesn't work on snobolds in the previous fight because snobolds are essentially immune to aoe and pest doesn't affect them as a secondary target). If the adds die too quickly, just retarget LJ and pest to keep diseases up. Your priority is keeping diseases up. Even if you're not GOD specced, pestilence is very important on this fight to keep dps on multiple targets. When the Infernals are up, ask your tank to tank them close enough to the boss to get pestilence, and also drop Death & Decay. They'll go down fast! But you'll need to use anti-magic shield and Icebound Fortitude, and you may need to run out if your health drops.
Fel Fireball: You might be asked to interrupt Fel Fireball. Your Mind Freeze is adequate to the task since your cooldown matches his cast timer, so do your job if asked. However, other classes are better for this. Rogues often get called on to kick, but I actually prefer Shaman to do this, especially enhancement since mana is not an issue (while energy and runic power used = dps loss for rogues, ferals, and DKs) and the only cost is a global cooldown.
Heroic: If you're on heroic, you know this: KILL Portals KILL Portals KILL Portals! Oh, and kill the volcanoes too. Both the portals and volcanoes are on a predictable timer, so be ready to switch. Just before a portal or volcano pops, I pestilence to keep diseases up, switch, kill the add spawning thing, then run back to LJ and pestilence again. Even if disease stacks drop, kill the freaking portals. If you let multiple Mistresses spawn, it's most often a wipe.
Your dps should be very competitive on this fight. Of course, you'll be beat by any competent mage since they get such huge buffs from spellstealing Jaraxxus' Nether Power. But otherwise, you should be a force to reckon with!