Unlike the medium mode, that doesn't require some change in tactics, this mode will really need concentration and flawless execution.
You will need:
-Lots of stamina: buff, flask, and two stamina trinkets
-3 pets: 2 Beetle/turtles, one Gorilla. Make sur they have last stand/intervene
THE FD THINGY: Many people complained about "FD not working". The answer is: If you FD then attack, your aggro won't reset. But if you FD, use misdirection, then attack, your aggro will. A dwarf died many times because of aggro pull to bring you this information :)
The goal of this phase is to kill Molgeim without your pet dying. So besides the obvious improved mend pet (about that, why isn't it anywhere in mist of pandaria? Totally unfair), and lots of stamina, you will need:
#showtooltip master's call
/target Durendil (player name)
/cast master's call
/target durendil (player name)
Basically, these two macros are needed to get your pet out of the blue runes, and they also get him out of melee range for 2-3 seconds.
/target stormcaller brundir
/cast silencing shot
To get Brundir out of the blue runes. Careful though, he'll try to melee you.
Use pet CDs on manual cast, when he's low on health, to soak a fusion punch.
Dispel Molgeims shield immediatly.
So if you're at full health, and Molgeim is nearly dead, you can prepare for phase 2.
Transition: Put yourself at 40 yards from steelbreaker, kill Molgeim, dismiss pet, summon Gorilla, heal, FD, misdirection, chimera shot, then go in silencing shot range from Brundir.
Kill Brundir while taking the least damage possible and remembering to heal your pet.
You want your pet to build aggro on steelbreaker for phase 3 while keeping aggro on Brundir and being in melee range of him.
When you interrupt Brundir, he will run to melee his aggro target. Run!
So, Steelbreaker's damage is easely manageable. But he will AoE, and put a debuff on you that makes you take +75% nature damage. In conjunction with Brundir, it really hurts.
So use aspect of the wild, and deal with Brundir:
-Lightning whirl: zomg interrupt it! Really. But silencing shot isn't enough for all of them, so use
/target stormcaller brundir
-Chain Lightning: up to 10K damage... and you need your interrupts. So:
*Feign death as soon as he starts to cast it on you!
*Keep aggro on your pet. He will melee him, so he won't chain-cast chain lightning.
Transition: Put your pet on defensive, and put hunter's mark and serpent sting on Steelbreaker. Then kill Brundir.
Dps Steelbreaker like mad. Once your pet's debuff is at less than 30 seconds, use widow venom (150K less heal). Then, once he's about to die, dismiss pet, summon 3rd one, FD and misdirect/give him hell. You now have one minute at 300% damage to kill him before you die, so pop all cooldowns, don't pull aggro, and don't get out of range (basically, don't do what I did).