I've been looking into Mega Blastoise and I was pleasantly surprised at what this thing is capable of. This is coming from someone that completely wrote-off Mega Blastoise originally.
Mega Blastoise is easy to understand as a bulky, harder-hitting (yes, harder than Life Orb) Greninja; or just think Swampert but actually threatening. Having STAB on all its attacks is extremely nice. It is extremely hard for non-stall teams to switch into because of its fantastic and powerful coverage but it also has a bit of trouble switching in initially. Mega Blastoise, while quite literally a tank, is not a wall and should not be switching into unresisted hits whatsoever. A Mega Blastoise in-battle with full HP is a daunting opponent and the opposing team will have to bend over backwards to take it out without losing a Pokemon. You really don't want to lose that by eating an Earthquake on switch, even if it only does 40%.
With that said, Mega Blastoise appreciates a VoltTurn core more than anything. Mega Blastoise can make a hell of a revenge Pokemon but it really wants to get in on Pokemon it can KO or threaten out without much risk and VoltTurn is the best way to do that. I personally like the following set on my own teams:
Blastoise-Mega @ Blastoisinite
Ability: Mega Launcher
EVs: 232 HP / 252 SAtk / 24 Spd
Modest Nature
- Aura Sphere
- Dark Pulse
- Hydro Pump / Dragon Pulse
- Rapid Spin / Yawn
Aura Sphere and Dark Pulse are absolute must have moves. They have far too much utility to run over anything else and they both get pseudo-STAB via Mega Launcher so there's no reason not to run them.
I'm sure the Dragon Pulse slash will raise some eyebrows, but it's there as an option over Water Pulse rather than Hydro Pump itself. Hydro Pump's accuracy is a little shaky and losing your Mega because of a miss is never fun, so naturally you might go for a base 90 Water Pulse, but I would advise Dragon Pulse instead. Water Pulse hits slightly harder with a base 135 with Mega Launcher and STAB while Dragon Pulse hits 127.5 base with Mega Launcher. You trade a bit of power and the confusion chance for completely different and unexpected coverage, only resisted by Fairy. It's also a nice work-around for not having to sacrifice a slot for a non-boosted Ice Beam just to hit Goodra, Dragonite, Salamence, and Garchomp. But again, Hydro Pump is much more powerful so you really have to weigh those pros and cons on your own team when choosing that slot.
As for Rapid Spin, I actually think that Mega Blastoise is the best Rapid Spinner in the game. A base 120 Dark Pulse scares away any Ghost that would dream of switching in is very potent. Even Excadrill has trouble with Gourgeist and Trevenant while Mega Blastoise tears them apart. So why the Yawn slash? Well on teams that don't need Rapid Spin, Yawn makes Mega Blastoise even harder to deal with by forcing checks out when they switch in bar Rotom and other Volt Switchers; but even then, you can potentially catch that Rotom after Yawn by switching to a Ground-type. But more on topic, the switching Yawn forces can be very obvious and extremely easy to take advantage of for Mega Blastoise with its wide coverage. And of course, if they choose to stay in, they fall asleep and Blastoise takes some damage but will live to fight another turn bar it being something like STAB Thunder.
While scary as hell, Mega Blastoise is no sweeper. What it tends to do, and what I believe it is good at, is blasting (hue) holes in the opposing team, while impeding them from sweeping, that will allow a teammate to sweep later on. I've had countless times that I swept with Scarf Garchomp, Scizor, Zapdos, or even stalled-away with my Gourgeist simply because Mega Blastoise took care of the things that stop them. In fact, Mega Blastoise seems like it might be best as a glue Pokemon like Porygon2 was in early Gen 5 but more of an offensive glue if that makes any sense. It might be too early to tell what its place in the metagame will be, but I look forward to finding out.