I wish that Sif were less annoying, that's for sure!
Anyway here are my preliminary notes for a strat, once we kill thorim I will add in all the usual stuff I do about what were the little things that we had to overcome to win.
2 tanks (one feral? see notes in phase 2 for details)
1 offtank DPSing type for the gauntlet adds
6-7 healers each with 1 piece of Frost resist gear
2 SPriests in full Frost resist
your best DPS each with 1 piece of frost resist gear
Gauntlet Team You need 2 healers, 1 tank, 1 offtank/DPS type and a bunch of ranged.
Now how many ranged you will need depends on your raid makeup. If you have some serious hard hitters, you probably don't need as many. You do need enough to really smash your way through as fast as pixelly possible. Tanks need to be chain pulling the adds, and engaging the mini bosses as soon as they can. The second mini boss should be engaged at the top of the stairs - we send the tank and one healer up there straight away while the OT and the ranged burn down the adds at the bottom of the stairs, and make their way up (dragging the adds with them). Smash the second miniboss and get to Thorim asap. If you are a bit slow, add more DPS into the gauntlet team until your balance is right. This will be different for each guild and possibly for each team!
1 aoe tank, the rest of the DPS (including the 2 shadow priests), the rest of the healers (about 4 or 5 of them). Same tactis as per normal, just theres less of you, so stay alive!
'Sif lose your illusion!
Ok, you engaged hardmode. Grats on the free achievement! Now the real work starts.
First up and most importantly is positioning to deal with all the extra frost bollocks Sif does.
Here is a picture from WowRaid
On the floor is a circle outside of which is the "blizzard zone" There is no reason for anyone to be caught here. Ranged should be in groups of no more than 3 to avoid death by chain lightning. Other than that, dodge the lightning by moving where the arrows say. Healers and SPriests MCing stay in their safe zones. DPS Thorim like a mofo!
MCing SPriests? Pfft Who needs bloodlust anyway?
Yep your 2 priests by this stage should be in control of a Warbringer each from the arena - which provides us with a constant heroism! YAY! Trust me, you will need them to get this done in time! The DPS of the Warbringers should be about equivalent to the SPriests output so no loss there.
Unbalancing strikes in this mode are much more unbalancing. You CAN get your furry friend to pop on PvP gear, and hit 625 resiliance. The math says this will mean your Fur tank will stay uncrittable (as there are no crushing blows) even through unbalancing strikes. This might work for you. Otherwise, get your 2 tanks taunting off each other every unbalancing strike and make sure your healers are on the ball for the cutover.
Once you get to about 8ish lightning charges, tank needs to start cycling cooldowns, get to 10 and you want the healers jumping in and helping out, by 12 make sure you have a rotation on keeping a CD up on that tank at all times, by 15 you will get oneshot.
Once you get thorim to 50% ish, get a soulstone on him in case of emergencies, and make sure you are ready to rebuff should something untoward happen.
Other general hardmode tips
Allocate your groups before you engage. Get everyone practising in the main room while one mob is sheeped or MCed. Make sure people are familiar with where they have been allocated that night.