Clearly by using Farsight.
Back on topic. I actually like this idea, Thrall making Garrosh warchief really was a CRITICAL FAILURE on his part, and if they don't put him to work to fix the mess then exile is appropriate in my books. A big problem with this whole warchief deal is that someone is always cleaning up after someone else's mess, it's very rare that they fix the problems during their reign (holy crap this sounds like the SAME situation as the freaking United States!).
Personally, I'm pulling for Thrall DYING in this expansion. It would make for an unexpected plot twist and a good quest/raid story. That or Wrathion revealing that he wants to be villain and utterly destroying Garrosh with bullshit dragon powers and then eating the Sha to become Omega Wrathion and just roasting everybody. Oh don't look at me like that, it would be hilarious.