Mega-mence needs to be Quick Banned.
1. It can set up a dragon dance relatively easily, especially with its huge bulk.
2. It is very hard to revenge kill due to its massive bulk + access to roost. About the only thing that can revenge kill mega-salamence after a dragon dance is scarf Greninja.
3. Although the Return/Roost/DragonDance/CoverageMoveOrSubstitute set is devastating, Mega-mence has other options that are also absurdly deadly, like MixMence or Special Salamence. These variants murder the counters to the first set (mostly kill steel types with Flamethrower/FireBlast). However, if you don't assume salamence is about to mega-evolve and dragon dance in your face, you can be very easily swept by the common DD variants.
4. You can tell by usage statistics that the mega-mence is overcentralizing. The meta is currently filled with:
-Mega-Mence (is broken)
-Skarmory, Heatran, Ferrothorn (Bulk to block mega-mence, carry shed shell to resist trapping by magnezone)
-Magnezone (trap the steel types that stop your mence from sweeping)
-Scarf Greninja (revenge OHKO's salamence after a DD with ice beam, clear out enemy fairy types for your mence with gunk shot)
-Landorus-T (intimidate Mega-Mence after a DD, set up rocks against enemy mence)
-Latias (bring your mence back to full)
-Rotom-W (resist mence before a DD, volt switch with lots of SpAtk investment to break a salamence sub)
-Mammoswine (set up rocks against enemy mence, ice shard to kill mence after some prior damage)
That is it. I saw a bunch of competitive matches, and it was so common to see people going against each other with 5/6 pokemon on their team being identical.
Summary: 1. Easy to set up. 2. Hard to revenge kill. 3. Has other options to kill counters. 4. Over-centralizing
Please quickban. It has the same problems as mega-kangaskhan.