First and foremost, I'll tackle the teambuilding process. I wanted to use Swords Dance Mega-Mawile as it was released recently and it seemed to be incredibly threatening to almost all opposing teams on a consistent basis, rewarding aggressive play such as double-switching in and getting a Swords Dance up in a tight spot. With this Pokemon at the center of my team, I felt that I had a lot of possibilities when it came to my next team member. I could go immediately with direct support such as Stealth Rock or Spikes, in attempt to weaken opposing Pokemon for Mega-Mawile and potential additional offensive members of the team to win or simply get more kills, or I could go a more indirect route and try to add a synergetic attacker that could situationally lure some problematic Pokemon that could be irksome to the team. Ultimately, I decided to go with Expert Belt Four Attacks Greninja as my second team member because it functions as a solid lure and general offensive breaker that synergizes quite well with the aforementioned Mega-Mawile; the moves I elected to use where Gunk Shot, Hydro Pump, Extrasensory, and Hidden Power Fire and I feel this set is optimal as it hits Toxapex, Ferrothorn, and Mega Scizor -- things that, upon being hit, make having Thunder Punch or Fire Fang on Mega-Mawile less necessary, freeing up room for the more spammable alternative in Knock Off. Despite the order of my import (sorry for this), my third member was dedicated to fixing a pitfall of many Mega-Mawile teams I noticed: lack of a true Psychic check. So many teams think that because Mega-Mawile is a Steel type, it can be used to check things like Tapu Lele and this simply is far from the truth. Moreover, I added Stealth Rock Healing Wish Jirachi. Healing Wish is greatly appreciated for the first two members of the team and the defensive capabilities of Jirachi are greatly underestimated in the current metagame, so I took advantage of it in this slot to check various special attackers, most notably the aforementioned Tapu Lele, but also Magearna, Tornadus-Therian, Latios, and Nihilego. Then, a Choice Scarf user seemed necessary because I had two slower Pokemon and nothing to reliably revenge kill opposing threats, so I added on what probably is the best, if not then surely top three, scarfer in the tier in Keldeo because it gave me a reliable Fire check and Greninja pivot while also fulfilling the revenge killer role. From there, the last two became fairly easy as I needed a Pheromosa check, Ground type pivot, secondary answer to Stall that would prove to be sufficient with Mega-Mawile, and perhaps a secondary win condition all in the last two slots. Double Dance Z-Stone Edge Landorus-Therian and Gengar emerged as solid candidates for both slots simply due to consistent offensive presence and fulfilling all of the things I needed them to while keeping offensive pressure up. I already had Z Landorus-Therian and Scarf Keldeo, so Gengar being Choice Specs seemed the most logical option to me and I went with it. The team, unfortunately, has a noteworthy weakness to Tapu Koko, but you can manage it with aggressive play despite it admittedly being an uphill battle.
Going into the specifics, the Jirachi spread was initially almost full SDef, but I needed Speed in order to creep Smeargle on opposing Sticky Web teams, so I added enough for that and then continued to creep until I outpaced Modest Tapu Lele, Adamant Zygarde, Mamoswine, Adamant Landorus-Therian, etc. which can all potentially come in handy. The Mawile spread assures that I live Sash or Jolly Scarf Dugtrio Earthquake 100% of the time from full while also outpacing standard Skamory on stall by 1 point (a few other things of note are living Protean LO HP Fire and non-Ash Specs Hydro from Greninja very often, living two Scarf or Shed Shell Psychic from Tapu Lele almost always, and always living 1 Earthquake from full from the rare TankChomp). The remaining spreads are all straightforward, but I elected to go with Jolly on Landorus-Therian as Mega-Mawile, Healing Wish, Gengar, etc. should be enough for Stall as is, so the extra damage is more of an unnecessary safety-blanket and outrunning Scarf Pheromosa at +2 is pretty much necessary. The moves on Greninja help set up for Mega-Mawile and Landorus-Therian, Gengar opts to not run Trick on this team so that it can pretty much hit everything not named Chansey hard in the tier and this plays right into the general dynamic of the team as weakening things early game leads to sweeps late game, and I do not use Stone Edge on Keldeo, despite the slight Volcarona "weakness" simply because Sucker Punch from Mega Mawile is doing 65% minimum to opposing Volcarona and there is no way I will be near full with Keldeo when potentially taking a boosted Giga Drain, so Keldeo + Mega-Mawile should do the trick easily enough (plus, Scald is such a good move, so why bother dropping it when you can run dual Water STAB n_n).
Overall, I think this team is quite solid and it has Keldeo + Mega-Mawile! :)