This is a balance build that I made a few days ago. It centers around the core of Mega Venusaur + Tapu Fini, which is pretty cool; Tapu Fini covers Defogging, Heatran, and Ash-Greninja, while Mega Venusaur covers electric and grass-types such as Tapu Koko and Tapu Bulu for Tapu Fini.
The spread I used for Mega Venusaur is a bit weird, but still does pretty good as an offensive tank. Yes. This is what I set up Mega Venusaur for (at least in my build): powerful, yet still bulky. HP Fire is ran to hit steel-types (not running earthquake since Mega Venusaur is intended to be a full-out special attacker, at least in my book), Giga Drain provides recovery outside of Synthesis, and Sludge Bomb is Mega Venusaur's main STAB move, it's actually pretty powerful and can force out anything not named Magearna, but even then, it hates HP Fire.
Tapu Fini is the Defogger for the team; she helps with removing entry hazards, and to an extent, dismantling passive teams. Defog and Taunt are the two main non-attacking moves she runs on her set, with the former providing HUGE support to this team, while the latter stops stuff such as Toxapex from using their recovery moves, attempting to annoy the opponent via Toxic, and/or setting up. It doesn't need to run anything more than its STABs in terms of attacking, because it's not built to be offensive. She is the trash-picker for the team, or to put it more accurately, the Defogger.
Heatran is the main SR setter of the team; while slow, it's able to sponge hits from the psychic- and flying-type attacks that Mega Venusaur HATES, while Mega Venusaur covers for everything else, from Ash-Greninja to Tapu Fini. Lava Plume its Heatran's main STAB move, which is really nothing special, although the ability to burn those annoying physical attackers that most of my team hates is decent. Earth Power is there to hit stuff such as Magearna because Garchomp is too shaky of a check imo, while Mega Venusaur not running EQ means it doesn't do very well against Magearna, so this is here to check Magearna for the team.
Garchomp is the power of the team, it's not only the secondary SR setter, but it can pressure common defoggers thanks to its coverage and SD. Earthquake does super well against a lot of common mons, and Swords Dance allows it to set up and overwhelm physical tanks with ease. Stone Edge allows it to absolutely destroy those annoying defoggers, such as Tornadus-T, and provides vast amounts of neutral coverage against everything else. Not to mention that it does decently well blocking Volt Switch. Unfortunately, I had to run Rocky Helmet because both of Volcarona's sets require a Z-Crystal for their item, and you're only allowed one Z-move per game. Nonetheless, Rocky Helmet in tandem with Rough Skin can absolutely screw over mons that require contact moves to do damage, such as Mega Lopunny.
Volcarona pairs decently well with the Mega Venu + Tapu Fini core, seeing as it appreciates Defog support from the latter and the former's ability to smash some of Volcarona's checks and counters, such as Heatran. It supplies the team with a way to check Kartana, Tapu Bulu, and Mega Mawile, all of which can prove problematic for this team to deal with, largely because I'm not running any physical defense investment on the former, while the latter gets utterly destroyed by the former two, while it has to watch out for the latter carrying Thunder Punch.
Gliscor is a bit of an "oddity" on this team, but it still does extremely well as the secondary set up sweeper. SD is for setting up, while Facade gets boosted by Toxic Orb's poisoning, allowing Gliscor to do MASSIVE damage against anything that's not a rock- or steel-type, but none of them want to think about eating an EQ from Gliscor at +2 attack. Its role is the secondary set up sweeper that hits extremely hard and takes a lot of hits fairly well.
Here's the PokePaste!