In the ramparts instance on the last boss, when you kill him and his flying dragon, if you click on the horn before the chest is spawned, he will come back again and your easily able to kill him for double the rep! you can keep doing this until you wait too long and the chest spawns. Works in both heroic and regular ramparts!
With regards to the horn, The horn was recently added to the ledge of the last boss's spawning point in Hellfire: Ramparts, due to the multiple strategies that are used to kill him, and with some of them causing him to despawn incorrectly preventing a respawn.
Vazruden's War-Horn - Objects - World of Warcraft