Just wanted to chime in as an Outlaw Rogue, After 3 times getting teleported to the Firelands entrance, I ran without using Killing Spree and I finally was allowed to beat Rag to get the achievement.
However! I did -not- get the option to loot the chest at the end, that continued to lock me out of any rewards.
I will give this a go with out killing spree and see how things go. Thanks for the suggestion!
Happened to me. not getting any credit for ragnaros kill due to this bug. Very frustrating.
Same, I keep getting teleported to the 10man firelands instance.
The response i got from Blizzard is this is a known issue with Combat rogues and they are working on it. Something to do with Grapple and Killing spree i believe. They suggested trying another spec, or I just did it with another toon after to much frustration my self.
Fear not, my fellow rogues, I have a solution. After reading comments on WoWhead and here, I can confirm this can be completed on a rogue as long as you are in Assassination or Sub spec, or you are dead from the beginning of the fight before Rag does Splitting Blow. It took me 3 tries but I was able to complete it after I switched specs from Outlaw to Assassination.