The previous team was centered around a bulky Bold Manaphy w/ RD + TG + Rest. Although I love this set, it's near impossible to make it work in the current meta with Volcanion, Gastrodon, Jellicent, and Seismitoed floating around. It also needs extra help against Serperior, Ferrothorn, and Amoonguss. So, instead I opted for Sitrus Berry 3 attacks.
Actually, a very similar team was played last week in SPL. Replay here:
https://replay.pokemonshowdown.com/smogtours-gen6ou-675819
Identical skeleton but one team had manaphy > volcanion. This team of mine is basically a mix of those teams and my old one. I'm using Thundurus-therian because it really grew on me when using that team. I was using Choice Scarf but w/ HP Flying. However on this team I'm using AV > Scarf because the extra speed doesn't seem needed while being able to comfortably switch on Rotom-W is huge. HP Flying gives you the ability to make plays such as baiting serperior, amoonguss, or mega venusaur and weakening them (or 2HKO) after rocks.
Using Timid allows me to use Modest on Manaphy, and Energy Ball manaphy will take care of water/grounds that Thundurus struggles with. Thundurus w/ Focus-blast & Tangrowth's HP Fire (& Mega Tyranitar w/ Fire Punch) all have a chance at weakening Ferro for Manaphy.
Clefable is Trick Barb for Excadrill. As the replay shows, Excadrill is quite annoying for this build archetype, especially if it's sand rush. Excadrill is Jolly for the same reason. With all this said and done, you still have Mega Tyranitar in the back threatening a sweep. Or it can be used early to put dents onto Clefable, Gastrodon, Ferrothorn, Mega Venusaur, etc. etc. for the rest of the team to shine. 16+ SpD on Clefable allow it to take Modest +3 Manaphy so that Moonblast can put in range of T-bolt.
Here's a quick replay of how the team's splendid synergy functions:
https://replay.pokemonshowdown.com/gen6ou-1806056903-o6b18fy40uazs3p2yynzg1k3bd2d5n6pw