At level 19 you have all these great aggro holders.

1. Always have Righteous Fury on.
2. Use Hand of Reckoning for pulls and during the fight.
3. Use Righteous Defense if one of the shamans get into trouble

I had similar problems in the Chasm, and I must admit that using the addon Tauntmaster helped alot, if a toon grabs aggro just right click on their name, that activates Righteous Defense and voila the mob is back on you.

I believe I started ragefire chasm at around level 16-17 as I recall, it tough but once you get the hang of it, its quite easy