#Update.
Since patch 5.2, the Razorgore the Untamed encounter was buffed and is much harder now to solo it. The mobs are killing the boss much faster.
Razorgore the Untamed can be soloed at level 90. Here is how to do it.
Dismiss your pet. You can’t control the boss if your pet is up.
Rush inside the room and kill the three mobs that are standing on the left side of the room, near the orb. Click the orb. You will now control the boss. Go near an egg and press the “4” ability. Repeat this for all the eggs.
Don’t worry about anything else. The mobs that will appear will start attacking you but you will survive as their damage is low.
Once you have destroyed all the eggs with the boss, you can now kill him to end the fight.
The rest of the bosses from Blackwing Lair are very easy to kill.
You can get some companion pets and some nice transmogrification gear.