What it does
Meteor Mash is a Steel-type physical move with 90 Base Power. On hit, it has roughly a 20% chance to raise the user's Attack by one stage. The damage is already respectable on its own; the self-boost is a bonus that makes it progressively more threatening if you get it.
Steel coverage pairs cleanly with Fighting. Fighting hits Rock, Steel, and Normal; Steel hits Fairy, Ice, and Rock. Together the two types leave almost no common defensive wall untouched. On Lucario, both are STAB, so Meteor Mash hits harder than the raw 90 Base Power suggests.
In doubles, an Attack boost is worth more than in singles because you can continue pressing the advantage with partner support — speed control, redirection, or chip from a second attacker — and the opponent has to respond to two threats at once.
Who runs it
Lucario is the only significant Meteor Mash user in Champions, at 45% usage. Its Fighting-type moves cover one attack angle; Meteor Mash covers the other. High Speed and Attack mean it can usually act first, and an Attack boost on top of that puts it in range to threaten almost anything that doesn't resist both of its STAB types.
How to use it
Use Meteor Mash whenever you have a Steel-weak or neutral target in front of you — don't hold it waiting for a perfect moment. The ~20% boost chance is a bonus, not a plan. Treat it like a pleasant surprise.
Fairy types are the best targets: you get double damage and the best odds of a boost snowball carrying into the next turn. Unlike Steel Beam, Meteor Mash doesn't trigger Flash Fire, so you don't have to read for Fire-type ability users before clicking it.
Tips & strategy
When the Attack boost does proc, your opponent's most common response is switching out to break the momentum. Have a plan: what comes in, can you still threaten it, or does your partner need to cover?
Meteor Mash makes contact, so Rough Skin and Rocky Helmet will chip you back. Against targets like Garchomp, track your HP carefully — the recoil adds up across a long game.