Blizz has introduced quests inside ICC 10/25 for raiders to complete, this is fantastic imo, I have seen 3 of the 4 so far and managed to do all 3 on both 10 and 25.
My favorite is at Lady D, you have to keep Darnavarn alive during the boss encounter to complete your quest and get the gold/rep/badges and a chance at gems or a 264 boe.
A strat that has failed is only using 2 tanks and having them switch off on the boss and Darn as the debuff stacks, we found that this was a bad way to do it, Darn sunders you which can equal insta death when you take the boss ie 30k frostbolt with a 12k mele on 25% less armor = done.
There are 2 strats that work quite well in 25 and 10m..3 tanks..2 on adds/boss and 1 keeping Darn busy...last night we went with 3 tanks...First time: Prot pally/Druid on DW and adds and my warrior alt tanking Darn...it was going well except a sp in the group had done 90k dmg to him and was about dead when the sheild went down..we wiped it.
2nd go: Everything going great...we get a deformed add and our druid taunted to close to it and went splat, we kept going but when the pally took the add and I grabbed DW she insta splat me with 42k dmg while i was at 38k hp (yes I was 1 sec late with laststand:P)
We got it on try 4, try 3 went bad due to a locked killing Darn:(
The other strat is using a dk to keep him chained/kited..works well too:)
Good luck on Arthas Ted and Devolve!