ZamKart + MagChomp Balance
(Click on the sprites for an importable!)
This team with Greninja-Ash is built with the core of TankChomp as the entry hazard setter and lead Pokémon to lure other threats such as Hetaran, Excadrill, and Tyranitar-Mega, which can dent the team if left unchecked.
Choice Scarf Kartana is the speed control Pokémon, and it can also deal with Rain teams which have Manaphy, Swampert-Mega and Azumarill in their team, and alongside Toxic Garchomp, you can cripple them severely.
Tornadus-Therian is the Z-Move user with Flynium-Z, a really strong Flying-type STAB move, which can heavily dent or outright OHKO opposing Pokémon such as Kartana, Tapu Bulu, Garchomp and Tangrowth, among others.
Magearna is the special wall of the team, with Assault Vest it can deal with massive threats such as Tapu Lele, Tapu Koko, opposing Greninja-Ash and opposing Magearna.
Greninja-Ash is the main wallbreaker of the team, as with priority in Water Shuriken and with the option to stack hazards in Spikes, it becomes extremely difficult to check. Spikes is chosen as its last move so that Alakazam-Mega can sweep easily late-game without worrying about lower rolls on Magearna and all.
Alakazam-Mega, along with Kartana and Greninja-Ash, can easily overwhelm the usual defensive cores of Heatran + Clefable + Venusaur-Mega and Toxapex + Gliscor + Latias-Mega. The last moveslot is Shadow Ball to mess with Jirachi, Tapu Lele and Reuniclus without having to go to Magearna and chipping it down.
The EVs are pretty straightforward, Magearna has 32 Speed EVs with a Sassy nature to outspeed uninvested Clefable so that it stands a chance to beat Calm Mind versions (and Clefable as a whole) late game.
Tyranitar-Mega than Alakazam-Mega is another choice, but I prefer the latter because of the beautiful offensive core of Alakazam + Kartana + Greninja-Ash, which can mess with most balanced and even offences easily.