Mawile @ Focus Sash Brave, Hyper Cutter IVs: 31/31/0/0-31/0/0 EVs: 255/255/0/0/0/0 or 252/252/0/4/0/0. The extra 4 don't really matter either way. ~Sucker Punch ~Metal Burst ~Protect ~Swagger/Super Fang/Toxic/Guard Swap
Protect to scout if desired, then Metal Burst. A 303 HP metal burst will -i s*** you not- OHKO any 31 HP IV, 0 HP EV pokemon with 156 or less base HP. i.e. anything less bulky than a Snorlax.
As far as max HP IVs and EVs, it OHKOs anything with 125 or less base HP. (Kyurem-B/W and lower)
Afterwards, follow up with a priority Sucker Punch which picks off a good 15-20% HP. Good in case Sucker Punch didn't get the job done (blue moon occurance) or just for getting that little extra advantage on the next pokemon (also breaking multiscale and sturdy effects in the process.) Sucker Punch also OHKO's Alakazams and lops off a solid 50% of Mewtwo's HP.
The last slot is just to harass any opponents that try to set up or otherwise purposely use a weak attack in attempts to make your Metal Burst nonlethal. You can abuse Protect with Swagger to hope for flinch hax. Keeping in mind that Sucker Punch will still hit even if they fail to attack due to confusion. And the advent of Super Fang being compatible means it's an option for use against the 157+ base HP pokemon or to soften up an opponent that's trying to setup. Lastly, Guard Swap is now available as well. I haven't tested it, but it might be good if you know the opponent isn't going to be hurting you this turn. (Though it means you can't use the Metal Burst strategy as well under it's effects.)
Obviously unfortunately countered by confusion, burn, hail, and entry hazards. Which all break the Focus Sash effects. If the day comes where Mawile gets Sturdy, you can just give it Leftovers and protect for one turn to completely negate any damage dealt by said counters. As well as potentially abuse it on Wish teams. Mawile's fun and is incredibly potent when it comes to stopping sweepers dead in their tracks, but the day Mawile gets Sturdy will be the day it takes a mandatory slot on my team :>
R- Official Slowpoketail Vendor Official Mawile of the Pokemon Boards.