Solo run of the second wing of Dire Maul. Also called the Ogre wing. You need to have the Crescent Key from the East wing in order to enter this wing. To get to King Gordok you also need to get two more keys, the first from Guard Mol'dar and the second from Fengus's chest. This instance is for level 55-60 players.
My brother and I found that you can defeat the "Love Is In the Air" seasonal boss with just two holy paladins. My brother was kind enough to record his perspective as well so I was able to show both characters.
The seasonal boss is actually three bosses, each with a specific school of magic. One boss attacks with perfume, one with cologne, and the third runs around dropping clouds of both perfume and cologne. You are given two potions, one which makes you immune to perfume the other to cologne. Each character uses one potion and then tanks the corresponding boss while avoiding the clouds from the third. Once the two bosses are down switch your focus to the third.
This fight took a long time (around 15 minutes) because as holy paladins we don't do much damage, but we can keep healing ourselves forever.
Solo run of the first wing of Dire Maul. You have to finish this wing before you can enter the other two wings, unless you group with someone who already has the key. In order to get the key you must chase an Pusillin the imp around the instance and then kill him. In order to get to the final boss you need to have the giant tree in the courtyard kick down the door. He won't help you until you kill Zevrim Thornhoof for him. This instance is meant for level 55-60 players.
Solo run of Zul'Farrak for the achievement. If all you're going for is the achievement for completing the dungeon you can skip a few bosses. This instance is meant for level 43-46 players and is located in the north-western corner of Tanaris.