Moves
Water Spout is an extremely powerful spread attack when Mega Blastoise is healthy, especially when it has rain support. Fake Out gives Mega Blastoise useful flinch support and is very important for ensuring that Mega Blastoise helps guarantee the setup of your speed control move of choice. If either if these are replaceable for you, consider Water Pulse, which is Mega Blastoise's most powerful placement-ignoring move. It's especially useful for smashing Landorus-T and reliably hitting Sylveon when Mega Blastoise's health is low. You can opt for Protect to save Mega Blastoise from getting seriously damaged from strong super effective attacks.
Thanks to Mega Launcher, Dark Pulse gets the equivalent of a STAB boost, making it Mega Blastoise's most reliable placement-ignoring move. It also happens to hit important targets such as Aegislash, Gothitelle, and Mega Gengar for super effective damage. Aura Sphere has nearly perfect coverage with Dark Pulse, grabbing an easy OHKO on Bisharp, and even having a 12.5% chance of OHKOing Mega Kangaskhan with no residual damage.
Set Details
Mega Blastoise should usually max its Special Attack to ensure that it gets the KOs it needs to function, such as OHKOing Meowstic with Dark Pulse, or defeating Tyranitar or Mega Kangaskhan with a small amount of residual damage via Aura Sphere. Rain Dish is the preferred ability, because it allows Blastoise to restore some of its health on switch-ins when it's raining before it Mega Evolves.
The biggest decision to make for Mega Blastoise is its Speed, as it has numerous speed benchmarks to consider. If running Mega Blastoise with Tailwind support, run 76 Speed EVs and a Modest nature to outspeed Adamant Choice Scarf Landorus-T. If you prefer to use Mega Blastoise with Trick Room support, run a Quiet nature and a Speed IV of 0 to maximize its effectiveness under Trick Room. If launching off powerful Water Spouts is your biggest concern, you can run a Timid nature with maximum Speed, though note that you lose some power in the process, such as the ability to always OHKO Bold Meowstic with Dark Pulse, or KO Tyranitar or Mega Kangaskhan after a bit of damage.
Usage Tips
Mega Blastoise's placement really depends on your gameplan and the stage of the game. If you want to use Dark Pulse and Aura Sphere, you'll want Mega Blastoise on the sides to avoid getting hit by any of the opponent's three Pokemon. If your plan is to use Fake Out, sending Mega Blastoise in the middle can be a good idea to be able to target any Pokemon. However, if your opponent is carrying a faster Fake Out user, it might be wise to put Mega Blastoise in the corner and make it shift to the center. This way, Mega Blastoise conserves its Fake Out so it can use it the next turn without fear of getting flinched itself!
While Mega Blastoise works well to set the tone early in the match, leaving it until later in the game can still be a very effective tactic, because as long as your desirable speed control is intact, Mega Blastoise can become a deadly spread sweeper with Water Spout. Just note that if your opponent is carrying Mega Charizard Y or Ninetales, Mega Blastoise's late-game utility will be diminished due to sunny weather.
Mega Blastoise's ability to ignore placement and flinch with Fake Out is the cause of many switches and uses of Protect, so try to take advantage of your opponent's predictability by targeting the Pokemon most likely to attack, such as vulnerable Pokemon in the middle, as this can give Mega Blastoise an effective edge over your opponent.
Team Options
The most important kind of support for Mega Blastoise is speed control. Because Mega Blastoise's Speed is rather middling, it isn't able to fully take advantage of Water Spout or its placement-ignoring attacks without usually going after the opponent. For faster variants of Mega Blastoise, Tailwind is best suited for the job. Talonflame is one of the premier users of Tailwind and can help against Wide Guard with Snatch and slam Amoonguss and Ludicolo with priority Brave Bird. Crobat works as an alternative to Talonflame that is less worried about Quick Guard and is immune to Fake Out, while Whimsicott has plenty of unique support moves in addition to Tailwind.
For slower Mega Blastoise, Trick Room support makes the most sense. Gothitelle and Aromatisse are especially good teammates to consider, as they both have access to Heal Pulse, which can power Mega Blastoise's Water Spout back up. Meowstic in particular can use Snatch to defeat Wide Guard, opening up a chance to use Water Spout. If you're looking for pure bulk, Cresselia will work fine enough.
Note that Mega Blastoise, while a good user of Fake Out, is one of the slower users of it (especially on Trick Room variants), so pairing Mega Blastoise up with a secondary Fake Out user such as Hitmontop or Smeargle can lower the pressure to set up speed control for Mega Blastoise.
For Mega Blastoise that focus on cleaning up with Water Spout, Wide Guard needs to be addressed. Similarly, Fake Out Mega Blastoise can't stand the idea of Quick Guard blocking its flinching. As previously mentioned, Talonflame, Crobat, and Meowstic are great choices for Snatch users. Clefairy is another great choice, making Mega Blastoise bulkier with Friend Guard, and having the ability to use Heal Pulse to power up Water Spout again. Hitmontop and Hariyama are among the best users of Feint around, and both benefit from Mega Blastoise taking care of Aegislash.
Mega Blastoise's biggest problem are redirection Pokemon, which will attract its movement-displacing moves. Amoonguss is the biggest problem due to its fantastic bulk, but Clefairy and Togekiss are also problematic due to their resistances to Aura Sphere and Dark Pulse. Taunt Pokemon do a great job of shutting these tactics down, such as Talonflame, Crobat, Sableye, and Whimsicott. Ironically, these very same Pokemon that give Mega Blastoise trouble are great teammates, since they can keep Mega Blastoise's health up by redirecting attacks.
While Politoed brings typing redundancies in, it's an especially great teammate thanks to Drizzle. Under the rain, Water Spout becomes extremely powerful, and Water Pulse becomes a hard-hitting move that ignores placement. Politoed can also aid in checking Mega Charizard Y by switching in after it has Mega Evolved. If you're especially worried about Mega Charizard Y, Rain Dance users can also make for good teammates. Klefki and Meowstic stand out as the best users of Rain Dance thanks to Prankster and access to other useful support moves, but Thundurus can also work if you need a more offensive support Pokemon. If bulk is more important, Latias makes for a great user of Rain Dance thanks to its ability to effectively handle Mega Charizard Y.