Sheer Force + Life Orb

Mega Beedrill works as an immediately threatening wall-breaker thanks to its sky-high Attack and Speed. It naturally out-speeds most of the metagame, and can be overwhelming for some teams to handle as Sheer Force + Life Orb grants it enough power to 1-2HKO pretty much anything that isn't a Fur Coat wall, while also easing prediction as you aren't locked into one move as you would if using a Choice Band set. However, Mega Beedrill is rather frail. Being weak to Stealth Rocks and falling easily to anything that out-speeds it makes controlling hazards and proper scouting necessary.

-Isshako

You want to switch Mega Beedrill in safely on your opponent and then drop a train on 'em with your STAB and super effective moves. Keeping it healthy is important as it can survive some resisted and neutral attacks, but it can be worth sacrificing Mega Beedrill to deal heavy damage and keep momentum for hyper-offense teams. Pairing Mega Beedrill with a Knock-absorber and a Sap-blocker is ideal, as it wants to keep the Life Orb power boost and it is one of the juiciest targets for Strength Sap users imaginable.

Moves

Most of the move options do not make contact, which is great for avoiding contact-based effects like burns from Burning Bulwark and Beak Blast.

• Noxious Torque is a strong STAB that dunks on Fairy types like Mega Audino, Flutter Mane, and Arceus-Fairy. Gunk Shot can be used for extra raw power, but the accuracy and PP loss can be frustrating to deal with.
• Mountain Gale provides coverage to hit many of the Dragon types in the tier, such as Eternatus, Giratina, Miraidon, and Mega Garchomp. Magical Torque is an option if you want a more accurate move + hit certain Fairy-weak pokémon like Zamazenta or Greninja-Ash, but you lose the ability to OHKO Garchomp and won't hit Eternatus hard.
• Bolt Strike is a strong move that can OHKO non-Fur Coat Flying types like Celesteela and Ho-oh, while also allowing you to potentially 2HKO Fur Coat Water-types such as Arceus-Water and Dondozo if you hit them on the switch and they have taken significant chip damage.
• The fourth move slot is mostly dedicated to how you choose to deal with steel types. Pyro Ball is strong, doesn't hinder you like V-Create would, and provides something to potentially hit an Imposter hard if you're forced into the position to do so, but can be worthless in the face of pokémon with Fire-immunity abilities like Well-Baked Body and Primordial Sea.
• Combat Torque can hit certain steel types that Pyro-Ball can't, like Dialga-Origin and any Fire-immune pokémon, as well as hitting the likes of Mega Tyranitar and non-Imposter Chansey/Blissey hard.
• Headlong Rush is there for those very specific pokémon that would otherwise be both Fire-immune and aren't weak to Fighting, such as Aegislash, Magearna, and Necrozma-Dusk-Mane, but it is counter-intuitive as it's not boosted by Sheer Force and it will both cause the Def/Sp.Def drop and the recoil from Life Orb.
• U-turn can let you pivot away from Steel-Types or other walls. It also gets STAB, and hits Dark types like Greninja-Ash and Ting-Lu. However, U-turn doesn't get the boost from Sheer Force and will cause recoil from Life Orb, and losing out on the coverage of other options can make Mega Beedrill less threatening.

• Imposter Chansey and Blissey can tank and force out Mega Beedrill, especially if it has Pyro Ball.
• Aegislash, Magearna, and Necrozma-Dusk-Mane with a Fire-immunity ability can wall Mega Beedrill if it isn't using Headlong Rush.
• Fur Coat Water Types like Mega Slowbro, Dondozo, and Arceus-Water can generally wall Mega Beedrill as long as they stay healthy. Mega Slowbro especially can take hits from Bolt Strike.
• Strength Sap in general can shut down Mega Beedrill.
• Faster pokémon like Mega Alakazam and Regieleki can OHKO Mega Beedrill.
• Triage users like Mega Heracross can out-speed and OHKO Mega Beedrill with Bitter Blade.
• Refrigerate, Galvanize, and Aerilate users like Kyurem-Black, Zekrom, and Rayquaza respectively all can out-speed and OHKO Mega Beedrill with Extreme Speed.