This quest grants 350 Stormwind reputation as of patch 2.01

Soloable as a level 29 assasination/combat (11/9/0) rogue.

I did it this way myself:
Sneak all the way to the boss room and kill the normal mobs in the corner of that room. Then proceed to the pack at the small wooden area.

Sap Gath'Ilzogg and kill the 2 normal adds as fast as possible, it's easier if you make the dragon your focus target and kick its Flamestrikes, they deal a nice bit of damage. When you get the normal adds down kill the little dragon, he goes down fast as long as you keep interrupting its spells with kick and gouge. After you've finished with him vanish and wait for cooldowns (if you had to use a potion or similiar items), the adds take a while to spawn so you're not in a hurry. When ready just engage Gath'llzogg himself, preferrably start with a cheap shot, build up combo points. Use expose armor after 4-5 combo points and your damage against the boss should increase greatly. Then just spank him like he was a normal monster, gouge + bandage if needed. He should go down pretty fast this way.

This can be taken as a horde if you go to the poster as a zombie.

He and his pet have a respawn rate of 7 minutes

Soloed as a 23 lvl Paladin. With one death - compliment to my ""Oh, I have the lay hands ability? I forgot!"". Got the dragon first, died, respawned and killed Gath'Ilzog on the second time. The whole garrison of the castle went down in some 10-15 minutes.

Tips for a Paladin (Primarily Retribution spec'd):
Keep Consecration up, & use Hammer of Justice on the dragon whenever it's up.
Burn down the dragon until it dies, by that time Gath should already be at half health. Finish him off.
Heal when necessary.

Recommended buffs are anything that will put out the most damage. (Retribution Aura, Blessing of Might, Seal of Command *for Ret Paladins*, Seal of Righteousness for other specs.)
The main idea is to put all your efforts into putting out damage, rather than defense.

Soloed as a 27 Paladin. Used Gift of the Naaru (Draenei racial) before entering combat, & Lay on Hands during.

Soloed as lvl 26 Druid.
Here's what I did, I'm sure there are alternate ways to do it.
Stealth in and nuke down the dragonwhelp then shift to bearform, use enrage and then bash to stun him. Get some distance, shift to human, cast entangling roots. Heal yourself.
Shift back to cat and finish off Gath.