What Emile said... plus some.

Arcurion casts the ability called Chains of Frost, which roots players in place. The boulders are actually thrown by adds that are above players, on the sides of the canyon. The range of the boulder hit is what those circles indicate. You can't avoid the roots, but you should definitely be out of the circles as much as possible, so you don't get rooted into where a boulder hits.

As for Mannoroth and Varo'then, the biggest damage dealer in the fight is an ability called Magistrike Arc, granted to the player via the 'elven magic' from in Varo'then's sword embedded in Mannoroth. From what I can tell, it is procced from your abilities, so the best way to utilize it is to use instant abilities, no matter how small their normal damage is. Heh, isn't everything instant for a feral? Just do your best to keep your Berserk and Tiger's Fury going, spam abilities with low energy cost, and thanks to the short 'swing' time for a feral's white damage, it should be easy to scrape out 100k+ on this encounter. (I'm not sure how dots work with that ability.)

Thanks for this guide. I always like to read about how to approach fights from specific specs, especially when it's an alt that I love, but that I just don't know every trick for.