You will need Aviana's Feather - Item - World of Warcraft , you can get it from the Lunarfall inn dungeon quest.
After you have this item queue up for the coliseum and wait for the battle to start. Once the battle starts use the item and land on one of the towers. Wait for them to kill each other and if you have some kind of slow fall or glider use it to land safely and kill the last one (you don't need to kill him to get the title,last 2 will get the achievement, that means you don't need to have a slow fall, you can just sucide and you will get the achievement)