There's actually no reason for Porygon-Z with Adaptability to run Shadow Ball in Inverse -Tri Attack hits exactly as hard against Normal and Dark types as Shadow Ball (assuming they're not Rock, Ghost, or Steel to boost Tri Attack's damage), and there's no Dark/Normal Pokemon to push Shadow Ball past Tri Attack. It really ought to run Nasty Plot or Agility in that fourth slot, Choice Specs or no, or maybe Toxic or Hyper Beam. Something actually relevant.
I'll do a write-up on Mega Beedrill as a Check later, preferably after the set we're supposed to victimize is altered.
EDIT ok sure give it a thing that's good against Mega Beedrill in specific
Mega Beedrill is lightning fast, almost certainly running Protect anyway (Which lets it stay in on Porygon-Z if they both lead or otherwise end up in a face-off with Beedrill not yet Mega-ed, in spite of the Speed disadvantage), and has monstrous damage output and excellent momentum.
252+ Atk Adaptability Beedrill U-turn vs. 4 HP / 0 Def Porygon-Z: 250-296 (80.1 - 94.8%) -- guaranteed 2HKO
Not actually a OHKO, but it does so much damage that Porygon-Z really doesn't want to stay in, and if does stay in it leaves it open to be finished off with priority or by Chansey smacking it with a Seismic Toss. Meanwhile, if Porygon-Z does switch out, Avalugg is one of the only things in the game that can effortlessly accept the hit -and then your Special attacker/Gothitelle comes in, ready to scare it out or kill it.
Alternatively, Mega Beedrill can anticipate the switch and toss out a Knock Off, which almost nothing is going to be happy taking, and then U-Turn out, assuming the enemy has no priority/their only priority is Fake Out. (Just Protect through it) This is riskier due to the possibility of Porygon-Z staying in and taking the hit (252+ Atk Beedrill Knock Off (97.5 BP) vs. 4 HP / 0 Def Porygon-Z: 173-204 (55.4 - 65.3%) -- guaranteed 2HKO), but if Mega Beedrill has taken no damage, Porygon-Z actually has a surprisingly low chance of KOing Mega Beedrill
252+ SpA Adaptability Porygon-Z Tri Attack vs. 0 HP / 0 SpD Beedrill: 238-280 (87.8 - 103.3%) -- 18.8% chance to OHKO
since it will lose its Specs in the process, and if Mega Beedrill does survive it's free to just U-Turn for the KO/momentum. In fact, a player happy with risks -or who invests a little bit into HP (40 HP EVs means Tri Attack can't KO, barring a crit, or alternatively 32 in Special Defense if you really want to minimize investment)- can stay in even if they aren't sure whether Porygon-Z is carrying Specs or Scarf, because Scarf will do the same surprisingly low damage, leading back to the prior situation of Mega Beedrill going for the U-Turn after a Knock Off.
Of course, it's tricky to actually switch in Mega Beedrill, since Porygon-Z can do so much damage, or throw around Thunder Waves and hope to cripple Mega Beedrill, but a gutsy player can try to arrange to switch in on Trick, at which point Porygon-Z is completely helpless and must switch or be sacrificed if switching is unacceptable, and if Mega Beedrill is supported by adequately slow, reasonably (Specially) bulky switchers (Such as Lanturn) it's entirely possible to get Mega Beedrill in on Porygon-Z with zero risk, so it's not as if you must lead/sacrifice/anticipate the Porygon-Z switch-in and switch in Mega Beedrill at the same time, actually. Mega Beedrill as a check to Porygon-Z is not for the faint of heart though, nor obviously stall teams, but it's more functional than you might think, and Mega Beedrill is such an amazing hole-puncher in general that it's not like you're nuts to run it otherwise.
Things to watch out for
-Anything with Mach Punch will kill Mega Beedrill instantly, so if Porygon-Z has such for teammates, it's extremely easy to lose Mega Beedrill. Beware Breloom, for instance, which is a good teammate for Porygon-Z between Spore, priority, and laughing maniacally at faster Ghosts who might try to counter Porygon-Z through outspeeding and hitting it with Shadow Ball.
-Sucker Punch is almost as bad, though actual Dark types will be afraid to switch in on a possible Knock Off from 150 Attack.
-Life Orb-carrying Agility-using Porygon-Z will outspeed (After an Agility) and OHKO Mega Beedrill, so if you haven't determined Specs for sure, Mega Beedrill carries risk as the first thing you turn to when seeing Porygon-Z. It's fine if you switch in on Porygon-Z "fresh", but if you're manually switching in Mega Beedrill (Expecting Trick, for example) and instead it uses Agility? You're in trouble.
Sample set.
Beedrill @ Beedrillite
Ability: Swarm
EVs: 4 HP / 252 Atk / 252 Spe
Jolly Nature
- U-turn
- Poison Jab
- Drill Run
- Protect
EDIT2: Dropped a "2" in 252+ in one calc, making it look stupid, and forgot to give Porygon-Z an item for calcing a Knock Off. Fixed now. Also myriad minor fixes to aid clarity.