This has to be by far my favorite core that I've built for this competition! Mega Beedrill is personally one of my favorite megas, and its a ton of fun to built around.
Anyways, Beedrill was the 1st mon in the core, because its so fast I didn't necessarily need to run a Scarfer (although, I was trying to fit in this cool Toxicroak set, but my dreams were crushed when I saw that I used up my last slot and forgot Skuntank rip). He in itself can sweep Psychic entirely bar Victini, and be able to outspeed dangerous threats like Scarf Gardevoir, Scarf Togekiss, Scarf Chandelure, ect (basically it outspeeds base 80 scarfers by merely 1 speed point). U-Turn is used over X-Scissor to keep momentum. I put Knock Off on it over Drill Run because I like it's utility better, and it does more damage (in case like a nearly guaranteed chance to OHKO Chandelure, I'd much rather that take).
Dragalge is the 2nd mon in the core, and acts as a Special Wallbreaker/Handy Fire Switchin. With Choice Specs slapped on it Dragalge is insanely powerful, being able to threaten a bunch of mons with STABdaptability Draco Meteor (even mons that resist it)! HP Fire is to cover mainly Scizor, as he can threaten the team in particular, and Focus Blast is generally for Steel mons that resist both its STABs.
Next I have Nidoking as my Mixed Attacker/SR setter. Superpower gives it a new-found range of coverage it didn't have before, being able to hit Air Balloon Heatran and Cradily with little trouble (also not to mention is great to deal with Chansey). The rest of the set however is pretty self explanatory.
This is my own Amoongus set I tailored a while back for a Poison Core Challenge I did a while back (unfortunately I was beaten out rip). Dual Status is pretty fun, you can spore 1 mon, get it to switch out, and Stun Spore the switchin, I love it! Giga Drain is mainly a general Grass Attack so I can do some damage vs Water and namely Keldeo. Foul Play is to deal with mons that feel they can boost against it. Regenerator is a great ability, especially for recovery, so I felt Synthesis wasn't needed.
Gengar acts as my fast Special Attacker/Sweeper. The cool thing about it is that it can take on many roles/fill in holes where coverage is needed most. Here I made it LO to give it more firepower, considering it's speed is already at a high level. Sludge Bomb is preferred here over Sludge Wave because I think the 30% poisoning chance us much better than a mere 5 more base power. Dazzling Gleam is coverage for Dragon and Dark types, and HP Ice is to check Landorus I, Garchomp, and Dragonite.
Skuntank was chosen over Drapion for 2 reasons: 1, Priority (and a reliable Victini check), and Hazard Removal. I like to run LO to boost it's attacking power up a pinch, I personally don't see a need to run Black Sludge. What's really nice about it though is Aftermath, which allows it to deal 25% damage upon dying to a Contact move--pair that up with Sucker Punch, and it can help cripple many dangerous threats to the team! You could honestly run Pursuit over Crunch to Pursuit trap, but, to me I liked a general Dark attack that doesn't have a chance of failing/low BP (plus, I don't mind the -defense effect it often has, makes it easier to wear down certain mons along with Poison Jab).