Scyther is a crucial card that, unlike all other crucial cards in the run, we aren't guaranteed a copy of. However, the free booster packs you receive in the mailbox can be fairly easily manipulated after a soft reset. You can save in front of the PC, with the cursor on the mailbox, then soft reset, and mash A to get back into your file and into the mailbox screen (where RNG is frozen) with fairly consistent timing.

The best way to time what frame you get in the mailbox is by looking at the first energy ball on the title screen's position. For the Scyther manip, you want it to be about up-right of the "TM" symbol before you advance to the main menu, kind of like this:

This position leads to a cluster of 3 Scythers (as well as 3 Gyarados), in this order: Dugtrio, Scyther, Electabuzz, Hitmonchan, Scoop Up, Pinsir, Wigglytuff, SCYTHER, SCYTHER, Gyarados, SCYTHER, Gyarados, Gyarados, Computer Search, Magneton, Magneton, Zapdos, Kangaskhan

If you don't get the Scyther, however, you can chain the other mailbox packs to reach another Scyther option, since RNG only advances by opening packs on the mailbox screen. Going earlier is better than going later, as there's no good chains off of the Magnetons. For best results, you need to have Mason Mail 2 (Col+Evo) available; with current routes, this means doing the manip after beating Lighting/Grass/Water. The chains always start from Pack 1, the default cursor location. However, the three-Scyther cluster is the same to reach on pack 3 and 6 as well. (And, if you're doing Lightning Club first, you have to swap any mentions of pack 3 and pack 4)

My manip focuses on obtaining valuable Rares in Colosseum, as well as Common trainers (Energy Search, Gust of Wind, Energy Removal, Gambler) in Evo/Mys/Lab. If the manip doesn't include the third Colosseum pack, it'll say "Exit", and you may want to then re-open the mailbox to randomly open the remaining Colosseum. Once you've opened all Colosseum packs, feel free to exit the manip if you feel you don't need the commons from the other packs.

I don't know how easily people will be able to memorize this, or reference it during a run, but it greatly improves your odds of getting Scyther in one manip try, and can even improve your deck for the rest of the game (all 3 Scyther frames near the Gyarados lead into a Computer Search, and maybe a second Scyther!)

Original 2018 manip: https://pastebin.com/P7en4VQ6 New 2020 manip: https://pastebin.com/Rb3qFj0L

Kangaskhan (Nidorino) - no Scyther Dugtrio (Raticate) - no Scyther Pinsir (Kadabra) - no Scyther Kangaskhan (Arcanine) - 5, 2, 6, 3, 4 Dugtrio (Kadabra) - 5, 2, 3, 4, 6 Gyarados (Nidorino) - 4, 5, 6, 3, 2 Dugtrio (Seaking) - 5, 4, 3, exit, 6 Scyther (Kadabra) - 5, 6, 4, 3 Electabuzz (Oak) - 5, 6, 4, 2, 3 Hitmonchan (Nidorino) - 5, 3, 4, exit, 6 Scoop Up (Kadabra) - 3, 6, 4, 2 Pinsir (Charmeleon) - 4, 3, exit, 6 Wigglytuff (Nidorino) - 2, 5, 3, 6, 4 Scyther (Seel) - 6, 2, 3, 4 Scyther (Charmeleon) - 3, 6, 4, 5 Gyarados (Growlithe, PP) - 4, 3, 6 Scyther (Magikarp) - 3, 4, exit, 6 Gyarados (PlusPower) - 4, 6, exit, 3 Gyarados (Growlithe, Kadab)- 4, 3, exit, 6 Comp Search (Dewgong) - 2, 6, 5, 4, 3 Magneton (Magikarp) - no Scyther Magneton (Raticate) - no Scyther Zapdos (Kadabra) - 5, 3, 2, 6, 4 Kangaskhan (Arcanine) - 5, 6, 3, 4 Dugtrio (Raticate) - 2, 3, 4, 6 Gyarados (Revive) - 4, 6, 3 Snorlax (Full Heal) - 4, 3, exit, 6 Item Finder (Kadabra) - no Scyther Item Finder (Seel) - 5, 2, 6, 3

If you don't need Scyther, you can focus more on Dugtrios, Comp Searches, and Item Finders. Here's manipulation for doing that. This is done by mashing as fast as possible, instead of aiming for the triple Scyther cluster. (Still need to update this to be more thorough)

https://pastebin.com/ZDU7HHwG

Here's the raw workspace I had when making the 2020 Scyther / Gene First manips, to give you an idea of how I make them if you'd like to make them yourself: https://pastebin.com/QCgCYUY6

Original work not done by Raagentreg