Your idea is nice in theory, but in practice, it will not suffice to keep Mandibuzz in check.
Unfortunately, Mandibuzz can extremely easily deal with Choice Specs Strategem...
https://replay.pokemonshowdown.com/gen8customgame-1543593928
And it leaves the 1v1 against Meteor Beam Strategem with low health at the cost of insanely boosted stats...
And this relies on Meteor Beam landing, which, due to Mandibuzz's agile and cunning nature, is extremely unlikely to hit since Mandibuzz can simply fly over the Meteor Beam. As proven by the history of banned mons and abilities in the OU tier, extremely unreliable checks are not considered checks and hence do not justify broken Pokemon staying in the tier.
https://replay.pokemonshowdown.com/gen8customgame-1543598246
As this replay displays, not even the greatest of fan creations can hold the monster buzzard at bay. Clearly we need the super creation of Gamefreak to pen this menace in. So naturally I went to the mega evolutions. And yet even they were a failure. After doing some damage calcs we can see why it can never work.
252+ Atk Choice Band Mandibuzz Brave Bird vs. 0 HP / 0 Def Garchomp-Mega: 1431- 1501 (401 - 420.69%) -- guaranteed OHKO
Garchomp-Mega Stone-Edge vs. Mandibuzz: 0-0 (0 - 0%) -- possibly the worst move ever
So if some of the most powerful megas cant break the buzz with their super effective coverage who can??? Something with an even greater, perhaps primeval, power?
And here we are. The one check (and a shaky one at that, certainly not a counter as that would be impossible)
Primal

Groudon @ Life Orb
Ability: Drought
EVs: 180 HP / 252 Atk / 76 Spe
Adamant Nature
- Precipice Blades
- Stone Edge
- Rock Polish
- Swords Dance
So here's the calculation
Groudon-Primal Precipice Blades vs. Mandibuzz: 0-0 (0 - 0%) -- possibly the worst move ever
Oh. Well then. Lets say a quick prayer to the OU dictator for a quickban.