Are you quite sure that Survival of the Fittest works after calculating your defense? That said, people will tell you regardless to wear your best gear and not obsess over being crit.

Install TankPoints, open calculator and manually subtract 200 defense (not rating). Then pile on resil gear until the crit box goes back to 0. Also, there're several very useful posts in the feral tanking thread in the druid forum which give gear lists which are confirmed to work. I built my set based on them and in 2 kills plus a large number of wipes learning have never seen a single crit.

Wouldn't simple UBS damage eventually oneshot you without cooldowns? Eventually, it hits for around 50k, which means any hit right after this will kill the tank - crit or not.

Edit: Late night reading comprehension failure. Delete.

I believe the idea is that if you're rotating tanks, the total pool of usable cooldowns will be higher.

Very easy solution to this is not to tank him if you have adds on you. Your tunnel tank should be free to pick him up in the center when he lands. I just charge him on the dais to start the hardmode, hop down, get in position and taunt. I also wouldn't bother with a long-ish cooldown like shieldslam when he's up top since he has a damage shield that almost completely negates the damage (making it a wasted CD, one that you'll want when he lands to make sure the add tanks don't get him with their D&D, consecrate, etc.) You'll want to save your threat generation abilities after the taunt, once he's in position.

Also, odds are good you have at least 2 tanks for the hardmode, so I don't see why anyone would single-tank him. Warriors can pop shieldblock on every other unbalancing cooldown to remove crits from the table long enough for the next tank to taunt. As someone else mentioned, you need to start popping cooldowns for every unbalancing after about 6-8 charges anyway, otherwise the unbalancing + normal autoattack will kill your tank. He swings far too fast for the healing to catch up with an instant attack + auto attacks without cooldowns at that point (crit or not). I know I have parses where he's swinging every 0.6 seconds fully debuffed near the end. No amount of healing is going to keep up with that, especially when your avoidance also disappears with unbalancing strike - it's not just the crits that are deadly after unbalancing.

We also use cooldowns on the new tank that is taunting after the transition.

Our kill target is around 11-12 charges (doable with two MCed Warbringers) -- from six to eight charges onwards both the tank and the new tank pop or receive a cooldown on every Unbalancing Strike cooldown. With a three tank rotation (two tanks in arena, one tank in hallway) it's relatively simple if there aren't excessive deaths. We tend to lose a tank or two along the way, but then he dies and it's over.

We've killed Thorim hard mode in u25 yesturday.
After several wipes we've noticed, that lightning charges are dealing no damage (people were not able to die from charges during wipes).
Any idea, is this some planned Blizzard thing or it's some bug?
World of Logs - Real Time Raid Analysis
It was damaging controlled adds, pets but not players.
At same time, it seems, that charge was damaging players at normal mode pulls

Definitely a bug, i have seen it as well in 10 man once. No idea what caused it though.