Unconventional Stall
Hey people, as you probably know I'm known for using Grass, and I thought that would be a good opportunity to explain how well Grass can work defensively (even though most people would rather go for the offensive path instead with mons such as Breloom and Serperior). Its actually one of my favorite styles to use on Grass, and I can state it works very well, thanks to the great and famous defensive core (Mega-Venusaur and Ferrothorn), but in fact, it actually has a nice variety of options to use defensively which are often overlooked, that I will list and explain below.
You might be surprised at the variety of options for this playstyle ;)
Impossible to forget, Mega-Venusaur has great defensive stats and Thick Fat (which makes it a must on every Defensive Grass team). Access to moves like Synthesis, Giga Drain, and support moves like Sleep Powder and Leech Seed makes it very durable and makes it a nuisance to be taken down for several types. It also helps versus so many threats who would just demolish your team otherwise. There's isn't really much to be said here, since Mega-Venusaur is already well known for its qualities and what it is capable of doing.
Another must on defensive grass, Ferrothorn's great Grass/Steel type allows it to form a great defensive core with Mega-Venusaur, making it able to handle many mons (those with Flying and Psychic coverage + Kyurem-Black) that threatens Venu. Although it lacks reliable recovery, Leech Seed + Protect allows it to dish a nice amount damage and gives Ferrothorn a good sustain and durability. Ferrothorn is also a great hazard setter, and excels at setting Spikes, which are great for defensive Grass, especially when you consider that Leech Seed puts pressure and can act as a semi-phazing move.
The best speciall wall avaliable on Grass, its secondary Rock type allows it to sponge many Fire and Flying moves coming from the special side. This is extremely important for taking down threats such as Chandelure, Togekiss and Mega-Pidgeot, which would be nearly impossible to handle otherwise. It is also a great Stealth Rock setter (since you're probably using Spikes on Ferro already, and it helps saving a moveslot on it). It gets Recover and Toxic which are also amazing for the team.
You might be wondering how you are dealing with setup sweepers, and this is the answer. Prankster + Encore makes it able to notably check most setup threats in the metagame, forcing them to switch. Moves like Leech Seed and Substitute can be very helpful at times, and it acts as a great stallish support (and annoyer).
It is also your main answer to Mega-Sableye and can be very helpful on Dragon, Fighting and Dark matchups. Feel free to run a more offensive approach on it if you are having difficulties on taking down those teams. Stun Spore is also an option worth mentioning, which it can reliably put Mega-Pinsir in check assuming you have enough speed EVs to be able to outspeed it, even if it uses Quick Attack.
Rotom-Mow is probably your best pivot, and it can work very well in this playstyle. Grass/Electric typing gives you a better time against Flying moves that can threaten your team, and its your best option at checking Mega-Pinsir. Access to Will-o-Wisp and Volt Switch helps keeping momentum, and can cripple physical threats. That's a solid option to be considered, for sure.
Tangrowth is often overlooked, which makes it surely underrated. A pure Grass typing might make you think that this option is useless, but think again. The fact its not weak to Psychic and Fighting moves allows it to check/counter many threatening mons that relies on that coverage (Hoopa-U, Meloetta, Alakazam, and so on). Knock Off is also very useful, checking the Pokemons I mentioned earlier and it also has the very utility of taking your opponent's item (useful vs normal teams). You should be using Assault Vest mostly, or else you're better with mons such as Gourgeist instead (in case if you are looking for a physical wall).
Regenerator and solid stats for a defensive pokemon are also worth being mentioned. Another interesting fact is that Ground gets way easier to handle if you are using this, considering how annoying Landorus and Garchomp can be at that matchup.
Celebi has solid stats all around, access to Recover, nice support moves(Thunder Wave, Leech Seed, Healing Wish), and Natural Cure, which functions as a great status absorber. It works nicely with offensive sets too (Choice Scarf/Life Orb), allowing your team to have an way easier time vs Poison and Fighting, and it also makes it more unpredictable, especially considering you are going for a defensive style.
Ludicolo is a good option to be considered because of its Water/Grass typing, even despite you're mostly going for an offensive set on this one. Rain Dance + Swift Swim allows it to become a immediate threat versus Fire (assuming the opposing CharY is gone) and Ground teams, and it can also switch-in on moves such as HP Fire from weaker mons, should you lose Cradily. It also has a decent coverage (Ice Beam, Scald/Hydro Pump, Giga Drain, Focus Blast, etc). The ability to handle Gliscor fairly effectively is also appreciated, considering how much of a threat SD Gliscor can be to this team.
That's an interesting option, although you might not need it most of the time, thanks to the fact you'll already have Mega-Venusaur (probably the only real thing it can do that Mega-Venusaur struggles against is handling -Mega-Medicham/Mega-Gallade). It does an amazing job on sponging physical hits, and Will-o-Wisp + Leech Seed can be an annoyance for unprepared opponents. It can act as a reliable spinblocker too, but keep in mind most of the teams actually rely on Defog for hazard removal.
Threatlist:
Bug Setup Sweepers:
That's easy to understand: Both Mega-Pinsir and Volcarona can demolish your team once they have used their setup moves. You can check them with Whimsicott (do keep in mind its risky switching in) if they use their setup moves. Cradily can handlle those, but then you have to make sure they are at +0. Rotom-Mow can be a nice answer to Mega-Pinsir, but be aware Quick Attack still troubles you at +2.
Fire and Psychic Wallbreakers
Fire wallbreakers can pretty much 2HKO all your team (that includes mega-venusaur and cradily), and psychic wallbreakers are as much as trouble thanks to their fighting coverage, which also destroys your team.
Why it is not common:
The fact Grass has 5 weakness often leaves it vulnerable against many types, and it gives an even harder time versus disadvantage machups. Access to mons such as Breloom, Serperior and Stun Spore Whimsicott tends to lead Grass teams to a more offensive playstyle, thanks to the power and the utility of the mentioned Pokemon.
