In my experience, there are three main categories of moves that are really helpful for grinding Mega Shards:
- Former priority moves, such as Quick Attack, Extreme Speed, Bullet Punch, etc. These moves come out very fast and usually have a low cooldown time, which makes them well-suited to attacking crystals along a path in quick succession.
- Moves that have a large area of effect, such as Earthquake, Surf, etc. The large hitbox for these moves is great for knocking out big clusters of crystals that are close together.
- Ranged moves. This is a big category and I'm including basically any move that fires off a projectile (like Swift) or spawns the attack directly above the target (like Thunderbolt), so your Pokemon doesn't need to be directly next to the target to hit it. These aren't the most important, but it can be useful to have one as backup for any crystals stuck on walls, across a gap, behind an object, or any otherwise hard to reach areas.
As a side note, it appears that using a physically larger Pokemon can be marginally advantageous for hitting a wider radius at once (credit to Vinc2612 on Smogon for this observation).
The Mega Crystal cleaner I personally use is an Alpha Lucario with Extreme Speed, Bullet Punch, Earthquake, and Aura Sphere.
Lucario is a good option since it's easy to catch once you have access to Zone 20 and its level-up learnset comes with a set of useful moves by default. You'll mostly want to alternate between Extreme Speed and Bullet Punch, which have such low base cooldowns that you won't even need to worry about your speed stat (the minimum cooldown time is 3 seconds). Earthquake is for large clusters that are close together; if you don't have the TM, Lucario can use Bulldoze from its level-up learnset instead. Aura Sphere is a ranged move that doubles as a powerful STAB move for getting wild Pokemon off your back.
Rooftops in general are good spots for finding Mega Crystals, but here are some specific recommendations for areas to visit:
- Up the ladder next to Hotel Z
- On the rooftop of the mall of Rouge Sector 2
- Near Carbink on the rooftops of Wild Zone 9 (large clusters)
- On the back side of Restaurant Le Yeah (stuck on walls)
You can get up to 3 Pink Canari Plushes from Racine Construction to upgrade the number of shards you get per crystal. As a final tip, note that since Mega Shards are categorized as regular items, you can only carry up to 999 of them at once, so make sure you spend them before they're maxed out. Good luck!
