Solo'd Sartharion 25 a few days ago. Not very difficult, just a matter of practice.
Now, I remind people that LK raid locks work like Cataclysm ones, meaning you can get in in 10-man mode, kill the drakes (with a friend for the evil one), then switch to 25-man.
His Berserk is 15 minutes, so maximizing dps is the key.
Positionning: You want a positioning where your turtle won't get hit by the waves, and you won't get hit by his tail (stun: bad).
However, it puts you at risk of taking Pyrobuffets (+1000 damage per attack per stack), with increases damage you take by the small adds, if you go too far to the left.
You need less than 9K on Sartharion to kill him, so the whole fight is about the adds.
How to deal with big red things:
-If they aggro you but not your pet, FD despawns them. Works once in a while.
-If they get hit by the lava waves, Tranqshot them.
-If you kill them all one at a time, GL for surviving the Berserk.
-try to despawn what you can
-If there are 1 or 2 on you kill them
-if there are a few on your pet, don't mend pet or they'll come at you
-if there are 5-6 in all, it's time to FD/misdirect/multishot.
Remember, they run slowly, so kiting them is easy.
At 10% health, Sartharion gets angry. And a whole bunch of angry little adds attack you. So make sure you have no pyrobuffet on, and Rapid fires off CD. Then focus boss with everything you got, run to kite the small adds.