Drake of the South Wind is a mount that was added in Cataclysm as a raid drop, but it is easy to get as a solo player at this point as long as you have a level 100+ character. Anyway, the mount drops from a boss in the raid instance Throne of the Four Winds. This raid only has two bosses, and it drops from the last boss; Al'Akir.
The drop chance seem to be around 3%, and it can drop on either 10man, 10man Heroic, 25N, or 25H. So it does not really matter if you farm it on either difficulty.
The raid can be farmed once per week, so if you really want it, your best option is to use multiple character to clear the raid.