What class are you?
You can probably 2-man it if not soloing it.
The person with aggro on the boss won't be MC'd, and the other person can Line of Sight it behind the pillar
I 2-manned it, got ashes on 4th 2-manning run
I have found that the easiest, surest way to kill Kael'thas is with a group of 5, although it can be done with less (hunters and DKs have been known to solo it). The mind control is the only troublesome part of the fight, and what prevents it from being more easily soloable.
You can clear the trash to Kael in under 10 minutes, and need not kill the first boss, Al'ar. Make sure you have 1-2 melee classes who can equip the dagger for breaking mind control, and a caster to equip and use the staff.
The hardest part is getting Ashes of Al'ar to drop - the loot rate is still around 1%, so while the fight may be trivial, you may still never see it drop.
Go 2 (Or 3 if it's a bad setup of classes). I did it with a RL mate. It turned out to be pretty easy, getting it on first and second kill.
We now started farming it with a DK (another RL mate). Just have someone who can use staff / dagger. If noone can then just LoS the mind control by standing behind the statues :>
Otherwise go tradechannel and reserve ashes of al'ar & then just say you RR like 20k G.
Just go with 4 people and roll for it properly, or agree to a certain payment before you start. A ninja'd years old vanity mount isn't worth losing all your credibility on a realm for. To ninja it is just silly.