When I started, the first thing I knew was that I could probably drop Grass STAB on Mega Venusaur. With Roserade and potentially Tangrowth running it, I could use greater coverage on Mega Venusaur, which would allow me to put the hurt on Steel teams (as much as I could at least). I opted for a physically defensive spread to help take on attackers that really are quite troubling, such as Mega Scizor and Choice Scarf Heracross. The Speed investment allows me to outspeed Skarmory, to pressure it with Hidden Power Fire, and happily also randomly puts me above Thundurus with Chlorophyll active for the very niche case.
Ferrothorn and Cradily are standard. Just the defensive core all Grass teams really ought to run. Provides necessary entry hazard support and checks a lot of threats with their status moves.
With the most important Grass-types out of the way, I wanted to put in Roserade. Toxic Spikes support is unbelievably good in Monotype, and I knew that was something I'd definitely use. Hidden Power Rock with Technician will always OHKO Mega Charizard Y, which Focus Sash helps to enable. Extrasensory is useful for taking on Fighting-types, as Roserade's good typing helps it survive one hit from a lot of them. Leaf Storm is just for STAB.
Next I added the other half of the core: Tangrowth. I opted for a physically defensive spread alongside Rocky Helmet to give me another nice check to physical attackers. For example, it can switch into Mega Medicham's Fake Out and still take a High Jump Kick, which forces it out. Tangrowth is a really reliable check to Excadrill, as few Grass-types can switch into a Choice Band Excadril's Earthquake or Iron Head. I really liked Sleep Powder and Knock Off utility, so those were easy additions. For the third moveslot, I like the utility of Leech Seed, but Hidden Power Fire just might be more necessary to take on Mega Scizor. 56 Special Defense EVs lets Tangrowth survive one Lava Plume from defensive Heatran and hit back hard with Earthquake.
My last Pokemon is, perhaps surprisingly, not Breloom. I wanted Speed control, something the team dearly lacked, but I also didn't want to just use a Choice Scarf user that would probably just lack damage. It also didn't help that I already had Sleep Powder on Tangrowth, meaning Spore's utility isn't nearly as valuable anymore. So, I opted for what seemed to be the obvious choice: Serperior. I specifically wanted to run SubSeed Serperior, as Serperior forces many switches and Substitute + Leech Seed helps me actually beat Chansey + Porygon2 when in conjunction with Leaf Storm's absurd damage, as I now don't have Breloom to do so. Hidden Power Fire is just obligatory anti-Grass-type coverage, since they resist Leaf Storm and are immune to Leech Seed. It also hits Mega Scizor, which is always nice.
Oh yeah, I considered lowering Serperior's Speed to 352, since obviously there's nothing to outspeed at 353 or 355 right? Luckily I actually know my Speed tiers and am not a horrible battler and Pokemon player, so I know that Thundurus is at 353 and Scolipede is at 355. Bullet avoided! :)