Weedle : the not-so-weak Glass Bazooka
Overview
(Preface, I will mainly talk about Mega-Beedrill, because although base Beedrill is usable, it needs the mega to work properly. Also although this guide is mainly for classic, it can also work for inverted challenge, where M-Beedrill is far stronger, in that case ignore the remarks on its typing) Beedrill Mega, often seen as the "worst mega" (OK only in vgc), today I want to put it on the spotlight. Admittedly, it isn't perfect, and isn't able to carry 100% of the run, but I believe that with the good tools and support, it can wreak havoc.
Strengths
Mega-Beedrill hits like a truck, with 150 Atk and Adaptability, granting it incredible STABs. It also has a stellar 145 base speed, letting it outspeed almost everyone. Egg moves also provides it excellent STABs. Its passive, Tinted Lens, allows him to hit well the vast majority of the mons. Also it will be fully evolved very very quickly, always a plus.
Weaknesses
It ABSOLUTELY needs its mega evolution. Before gaining it, Beedrill will be far more slower and hit far FAR more weaker, so it will be very mediocre at best. Even with its mega, it stays very frail, especially on physical damage, so priorities are a menace. Its typing is also not good, without its passive and ability it would stupidly handicap it. Also even if it has a setup move, its bulk is so bad that it's dangerous to use it.
Recommended build
145 speed is more than sufficient to gave it Adamant nature. For the items, gave it whatever will make it hit harder : Protein, Soul Dew, Silver Powder..
Moves & abilities
For its moveset, the two main moves will be its two STABs moves, the others two are more niche. Noxious Torque and Attack Order are the stars of the moveset. They have great base power, perfect precision and no drawback. Most of the time they'll do the trick (considering the passive), there's not much to talk about. For its coverage move, it used to have Thousand Arrows, which was perfect, but now it has close combat. It's not a bad move at all, but it's significantly worse that its previous one. I would say you can either keep Close combat, or pick Drill Run at Lv47. Both works. And for its last move, if you have Victory dance you could try, the defense boost may prevent it from being OHKOed, and the Atk boost is very nice. But my personal choice will goes to Fell Stinger. If you successfully kill a pokemon with it, M-Beedrill gain +3 Atk, enough to kill in one hit every pokemon except bosses. And it can snowball, because it will be more easy to pull off a second fell stinger after the first one. It's risky if you're not certain to KOing the enemy, but this is a risk-taking pokemon itself, so I'll say it's worth a shot.
Strategy
The hardest part will by far be the early game, with the very late game coming close. You'll have to be lucky to quickly pull the Mega Bracelet, and then the mega stone. Base Beedrill isn't unusable, since its passive is Adaptability, so it will hit decently strong, but it's nowhere its level of violence as soon as the mega is available (and also it's slow and it will be resisted by too much mons). If you succeeded, then M-Beedrill will be able to defeat the vast majority of trainers and wild pokemons. Watch out for bosses, anticipate them by getting some Atk buffs with fell stinger before them. The two late game dangers will be the wave 195 rival and Eternatus. The rival will greatly depend on which type their starter is. If it's Grass type, they are very likely to switch it out. If Fire, it will dangerously attack, and if Water, it will normally attack. Unless you pull a LOT of buffs, soul dew.. you cannot one shot rayquaza. You'll have to either get one or two KO with Fell Stinger, or anticipate the Rayquaza and clicking Noxious Torque on switch in. For Eternatus, if you're mad you can try, but personally i'll strongly recommand to bring 1, if you can 2 pokemons to handle the final boss. M-Beedrill will get KOed too quickly.
Synergies
Choose in priority a pokemon that can handle early waves well, and one or two for Eternatus. You can also pick a Ground pokemon, being super effective against Steel, Poison, fire, in case M-Beedrill cannot pass them. Oh and if they're shinies, that's a nice plus.
Tips for beginners
Pull M-Beedrill out on pokemons that don't hit very hard. You can also weaken a pokemon with another pokemon, then switch to M-Beedrill and use Fell Stinger to guarantee the KO.
Tips for advanced players
If you can find some moves that it can learn with TMs, you can try another strategies.