1) Destro. Apparently Demo used to be viable but not anymore due to rebalancing early in MoP. But you could probably get away with the other specs if you really wanted to and the group was okay with it, it's just that Destro's AoE is hard to say no to and looks so sexy
2) GoSac, simply because not having a pet means no pets accidentally pulling during an invis run or running down the ramp to Gekkan etc. If you really want to the other Grimoires work just fine, I've seen people using pets in plenty of runs. GoSac just means one less thing to manage.
3) Legendary scales down like anything else, proc doesn't work, however it has nice sockets. Not exactly sure how the scaling works on the three stats (I've noticed that gear with only 1 secondary stat doesn't scale properly) but safe to say the cloak is a strong choice for CMs if not BiS. Just remember to change out your helm for a regular meta gem.
Also, if possible get gear with as many sockets as possible, ToT weapons are good because they have one socket plus the one from the Eye of the Black Prince (only legendary reward that works)
Most important things as a lock: always have RoF up for AoE pulls (90% of CMs), use Havoc with CB/Shadowburn where you can, and of course Shadowfury is the absolute shit (for best results coordinate a stun rotation with your party). Don't be afraid to drop an Infernal on some of the harder trash packs for the AoE stun as much as the damage. Also, your pet (sacced or otherwise) should be a Felhunter for the interrupt most of the time. Interrupts are important! Oh and Soul Leech is OP as fuck. Can eat AoE that lesser mortals need CDs for :P
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Only down side of that strat is people sometimes die after boss is dead because healer forgets about dispels :P
Running my first challenge modes this weekend. Late to the game i know but its was never a big thing for me til they said they were going away lol. My question is. Does the legendary meta not work in challenge modes? Also how about our set bonuses? I have a few Heroic warforged items in the bank and was wondering is they would be better to use over the set. But if the set bonuses still work then i'd obviously use that over the warforged stuff. KTT and Immers trinkets still the good to run with?
Lots of questions i know but im running with a very good group and would love to be able to compete for the best realm times.
Legendary meta is disabled, legendary cloak proc is disabled and so are all set bonuses. Trinkets you wanna aim for are DMC and Elegon trinket, ppm trinkets are garbage in CM. Alchemy trinket is also good if you have that.
Other than that aim for mastery gear and as many sockets as you can. Try to get hit capped without reforging into hit as it gets kinda weird/buggy when downscaled. I assume you can safely reforge out of hit but haven't tried it.