[OVERVIEW]
Mega Blastoise is one of the best wallbreakers of in the STABmons metagame thanks to its hefty Special Attack stat and a very helpful ability in Mega Launcher that boosts the power of Origin Pulse to a 165 Base Power move while also making its coverage options such as Dark Pulse and Aura Sphere to hit significantly harder. Mega Blastoise's access to Rapid Spin makes it helpful to teams that need entry hazards control. Mega Blastoise It also has above average bulk and a good defensive typing,(RC) which boasts boasting (eh, "boasting" is fine imo, makes more sense to say MStoise has the weaknesses than to say its typing does afaict) only 2 two weaknesses. However, (AC) Mega Blastoise does have its shortcomings, namely its lack of boosting moves and its lack of reliable recovery, which make it unable to break through dedicated special walls and more easily avoid being ("leave it more easily worn down" imo, that resolves the clunkiness even better and "make it unable to avoid being worn down" might not be correct)worn down over the course of a game. Mega Blastoise also takes up the a team's Mega Evolution slot, removing the possibility of running other very powerful and useful Mega Evolutions such as Mega Scizor or Mega Altaria (this is banned now, right?). Mega Blastoise's subpar Speed also limits its sweeping potential late-game, and, as regular Blastoise is not viable in STABmons, its item choice is restricted to its Mega Stone. Lastly, the Mega Evolution slot can be freed by replacing Mega Blastoise with other powerful Water-types such as Keldeo,(AC) which offers a secondary typing as well, and Starmie, which can do many of the things that Mega Blastoise can, while also including ("while also" implies that rapid spin isn't in the list of things blastoise can do) (ya good catch, but this is clunky because calling "having access to RS" something Blastoise "can do" feels awkward. "and also has" is a proper fix) having access to Rapid Spin.
[SET]
name: Offensive Support
move 1: Origin Pulse
move 2: Ice Beam
move 3: Rapid Spin
move 4: Dark Pulse / Aura Sphere (add spaces around hyphen)
item: Blastoisinite
ability: Rain Dish
nature: Modest
evs: 136 HP / 252 SpA / 120 Spe
[SET COMMENTS]
Moves
========
Origin Pulse is the only reason to run Mega Blastoise in STABmons, as Mega Launcher boosts its Base Power goes from 110 to 165, which is more even more powerful than a Water Spout at full health. Ice Beam is a great coverage move, as two of the three types that resist Water are weak to it, namely Grass and Dragon, allowing Mega Blastoise to hit some of its checks, including Amoonguss and Latias. Rapid Spin allows Mega Blastoise to clear entry hazards from its side of the field, and Ghost-types have a hard time switching into Mega Blastoise because of its access to Mega Launcher-boosted Dark Pulse and Origin Pulse. The last slot is up to the user's preference. Aura Sphere is nice to hit Ferrothorn, Mega Gyarados, and Kyurem-B for good chunks of damage, (comma) while Dark Pulse has excellent neutral coverage in conjunction with Origin Pulse and comes in handy against some common Pokemon that resist Water such as Slowking, Slowbro, Latios, and Latias. (doesn't reflect set order)
Set Details
========
Blastoisinite allows Blastoise to Mega Evolve. A Modest nature is necessary to have Mega Blastoise hitting as hard as possible, and it has no reason to run a Timid nature due to its mediocre Speed tier. The given Speed EVs allow Mega Blastoise to outspeed defensive Landorus-T with 8 Speed EVs. (no creeping in analyses, because there's no reason why that wouldn't lead to a clusterfuck of countercreep [yes that has happened in the past before this was made policy]. just move them to HP and make the benchmark uninvested Landorus-T imo, unless you have another benchmark with either no or full investment available) The remaining EVs are put into HP so that Mega Blastoise can live survive as long as possible, as it lacks any form of recovery.(s p a c e)Mega Launcher makes Mega Blastoise extraordinarily powerful, allowing it to OHKO a lot of common Pokemon in STABmons. There's also the option to run a faster Mega Blastoise with a spread of 24 HP / 252 SpA / 232 Spe, which would lets it outpace Adamant Mega Scizor.
Usage Tips
========
Mega Blastoise should be mainly used on offensive or balanced teams thanks due (ya ok, not sure if i would have done this but i can see the positive connotation feeling out of place here) to its ability to break through walls, making it easier for its teammates to sweep. Mega Launcher-boosted Origin Pulse is one of the most powerful special attacks in the metagame, so it should be spammed as much as possible. Mega Blastoise is able to force out many common Pokemon in the tier such as Heatran, Weavile, Tyranitar,(AC) and Landorus-T,(AC) which gives it the opportunity to remove entry hazards or deal damage to the Pokemon coming in on the switch. Mega Blastoise doesn't appreciate being burned, but it isn't game ending. Don't forget to use Rapid Spin, as getting rid of entry hazards is always nice and because Mega Blastoise can easily take advantage of the many switches it forces to clean up its side of the field. (you already said this above, i don't think it's necessary)
Team Options
========
The biggest problem that threats to Mega Blastoise has is are Chansey and other specially defensive Pokemon, so a physical wallbreaker is a really nice companion for Mega Blastoise. Some options include Terrakion, Aerodactyl, and Infernape. Fast Electric- and Grass-types also threaten Mega Blastoise; Thundurus,(RC) in particular,(RC) is troublesome, so Pokemon that can deal with it such as Excadrill, Kyurem-B,(AC) or a and Thundurus-T make for great partners. Grass-(AH)types help to absorb Electric- and Grass-(AH)type moves that can really deter trouble ('deter' doesn't exactly fit) Mega Blastoise: Ferrothorn makes for a nice defensive partner, while Serperior can make for a good offensive partner. Flying-types such as Talonflame, Thundurus, and Landorus-T make for good partners,(AC) as they appreciate Mega Blastoise's ability to remove Stealth Rocks Rock,(RC) so Talonflame, Thundurus and Landorus-T can make for good partners, and they can also deal with some some of Mega Blastoise's Grass-type checks in return. Pokemon with access to moves such as Sticky Web and Thunder Wave that can help boost with the problem of Mega Blastoise's Speed are very helpful as well,(RC) a Pokemon that can set up Sticky Webs or have access to Thunder Wave. Scizor is a common Sticky Web setter, (AC) while a Pokemon like Thundurus can use Thunder Wave to help support Mega Blastoise.
[STRATEGY COMMENTS]
Other Options
=============
A RestTalk variant of Mega Blastoise can work very well on certain teams, as the this combination of Rest and Sleep Talk gives Mega Blastoise a great advantage in survivability staying power (eh, this is still somewhat awkward and i don't think this is entirely what "staying power" means [endurance]. rephrasing it to "greatly boosts Mega Blastoise's longevity" is best imo) and can still provide without fully compromising its (double check content etc, but RestTalk def. doesn't "provide" wallbreaking potential) wallbreaking potential. Other viable moves that Mega Blastoise can run include Roar or and Dragon Tail, which provide phazing support for the team. It is possible to run a defensive EV spread in conjunction with Scald,(RC) and (eh, this seems pointless) Dragon Tail or Roar, Rapid Spin, and Ice Beam (it's possible i misjudged your intentions with this fix, but this was my best guess). A set based on Mirror Coat is an alternative, although having it has a pretty small niche.
Checks and Counters
===================
**Special Walls**: Chansey walls Mega Blastoise all day(finally, a colloquialism!),(AC) and because of Mega Blastoise's inability to set up, it really has no way of getting around Chansey or other special walls that resist its coverage moves.
**Grass-types**:(s p a c e)Serperior, Mega Venusaur, and Whimsicott can be very annoying to deal with for Mega Blastoise to deal with, (comma) as they can use Spore to put Mega Blastoise (keep "it" pls, repetition) to it asleep. They can also and can hit hard with Giga Drain and Leaf Storm. Ferrothorn also falls into this category; if Mega Blastoise does not have Aura Sphere, it can be really difficult to break through Ferrothorn.
**Electric-types**: Both Thundurus formes can check Mega Blastoise pretty well, as they can OHKO it with a Life Orb-boosted Thunderbolt; other fast Electric-types such as Mega Manectric (remove hyphen) and Raikou are also a nuisance for Mega Blastoise to deal with. While Electric-types cannot switch into Mega Blastoise safely, they force it out once they are on the battlefield.
**Water-types**: Water-types resist Origin Pulse and thus can give Mega Blastoise problems. Some of them, such as Azumarill, Rotom-W, and Gyarados, can beat it regardless of the coverage moves it is running, while others, such as Slowking and Slowbro, can do it depending on which moves Mega Blastoise has. Manaphy also only takes neutral damage from any of Mega Blastoise's common coverage moves and can set up using Tail Glow and then fire back with Energy Ball.
**Bulky Dragon-types**: (dash to colon) ("dragons" is acceptable actually, along with "ghosts" and "fairies". that's not to say i dislike this fix here because header consistency, but yeah for future reference) What comes to mind is Kyurem-B. It can tank hits from Mega Blastoise even after Stealth Rock damage and can hit back hard, KOing Mega Blastoise ("it")depending on the set it is running. There's really nothing in Mega Blastoise's arsenal that lets it win this one-on-one situation, so a partner to check this threat is important. Other bulky Dragon-types that can threaten and potentially set up Calm Minds on Mega Blastoise are Latios and Latias.