I just did my first ever 4 challenge modes last night and got gold in all of them with relative ease with an avoidance build. Our dps warrior was even using an int flask and we still had plenty of time left. :-)
Considering in 5.4 you get Riposte, which works in Challenge Modes, avoidance is doable and definitely the best way - damage wise. Pre-5.4 it was mastery, but can't go wrong either way...
Stick with avoidance based build... Sucks being brought down to 19-20% total avoidance, but the class revolves around Riposte now
Had no trouble getting golds in avoidance build (using my normal raiding gear - so no working meta, etc). With the pre-nerf vengeance levels still active in C-modes my damage was pretty ridiculous considering the gear scaling.