In terms of gearing, avoidance is likely better than mastery due to the riposte conversion.
What matters more is just finding gear with slews of sockets. Engineering trinket is pretty solid too.
Just make sure you plan out your CD usage for each of the 10 waves, each of the major waves should have either a SW/LS/recklessness. Also, aoe fear is pretty solid for damage reduction (i'd minor glyph it though).