I ran mastery-arcane build on 1 challenge mode I've done so far and it sucked. Aswell as Arcane spec overall on trash. Yea the boss-damage is insane, good, steady, raid-boss-damage. But Arcane/Fire is useless on trash. I tried dotting up and spread with Fire but it didnt work, Inferno Blast has way too long CD, so does Combustion, you don't have any viable abilities to use on those trash mobs at all, every massive AoE mage ability is weaker than for example Howling Blast.
I tried going Arcane with Nether Tempest, tried to dot up with nether tempest and run Arcane Explosion + Barrage - same crappy results.
So I suggest going Frost is the best choice for Challenge Modes. Once you get that crit cap, go all crazy with haste and win. I'm about to try it out myself at the end of the week as we've only done Silver and I was Arcane so far.
I've done all the gold dungeons except two, for those saying arcane is useless, rubbish. I've never gone frost for any and our group has cleared with good times. Arcane does fine damage on trash as long as you know what you are doing and single target output is way higher than I could achieve with frost. Use frost armor for extra haste and maximize your cleave with arcane barrage and living bomb spread with glyphs fire blast. Make sure you maximize your cool down usage as well, I.e. arcane power. First pull of shado pan is a good example. Invocate + prepot + arcane power with a well timed frost bomb and arcane explosion spam sees me pulling 250k to 300k dps during that big pull, and you can always barrage at six stacks due to ae procs. Yes of course frost does better trash burst dps, but my impression over all our runs is that the single target dmg from arcane probably puts them on par from a time point of view and I was just far more comfortable with arcane as its what I raid with.
Friends and me began challenges a couple of weeks ago and so far we are 6/9. Like an evening for one instance, maximum (and not playing every evening).
Some are really easy, some are nasty.
The nastier for us is Stormstout Brewery.
Just to say that as for me I play frost during challenges, for many reasons. And I just reforge my ilvl 495 stuff as I would do in raid, then go to challenge.
At first, I tried to maximize my stuff by making such calculation (bla bla bla which is best bla bla bla which cap when I am scaled down bla bla bla). But really, it doesn't matter.
Your team is fine, your brain is fine, you experience the instance and do a couple of tries, it's down.
More and more, it appears that calculations and itemisation are just changing infinitesimal percents in raids. The player and the experience makes the difference. It is all the more true in challenge.
So don't spend more time than necessary in theorycrafting challenge modes, that is my advice.
If one runs Challenge Modes as Frost, could it potentially be advisable to stack Crit (over haste) until you get as close to the 23% softcap as possible, due to the importance of aoe pack damage (rather than single target)?
Or would Haste still be king?
Thats what I did, but it wasn't the most practical way to do it, my group just brute forced our way through challenge modes, running double mage fury war as our dps so the aoe our group did was outstanding, but the single target I did was pretty lackluster as frost with crit gems, as I was still gemmed from prenerf fire.