Hi! I'd like to give it a bit of feedback after I've read it. Glad to see new guide makers 🙂
There are technically better and earlier places than provided above, with quicker methods. However, I'll go step by step commenting on it. This is purely my opinion, I think the guide completely works as it is.
- Black Medallion (highly recomended)
^In my opinion this should just be obligatory, as hunting a pokemon's HA without Black Medallion is painful beyond belief.
- Low to medium level Rattata with Sucker Punch move (easy and fast pokemon to get)
You can also use Pursuit or other Dark Type moves it learns such as Bite (level 10, Pursuit at 13). I'd still recommend using a Trace pokemon in the second slot equipped with a Smoke Ball as it instantly shows the ability and we can flee if it is not what we are looking for. In the first slot we can have a Fainted Synch (equip a Toxic Orb and spam Flash or any non-damaging move until it faints quickly).
^reediting this: I'm aware getting a Trace Ralts can be a butt pain in the early game, still feasible to find in Love Island if you complete its requirements before Johto though.
- Smoke ball item for rattata (usually dropped by wild grimers, koffings and weezings and helps you to flee from unwanted wild encounters)
To remark here: Smoke Ball is obtained with extreme easy while surfing in Celadon City and using Thief (TM), Covet (some pokemon learn it via level up), Trick (Kadabra line and more) on wild Grimers and Koffings. They're tier 1-2. We can ignore Muk as it does not give them. However, keep in mind Grimer and Muk have Sticky Hold, so you can use a Pinsir with Mold Breaker (or any Mold Breaker pokemon) with Thief learned, such as Axew/evos or Cranidos/evos. Keep it to Pinsir so newbies in Kanto can also get it via Headbutts (when they're enabled again)/Safari.
- Acess to Trainers Valley (not 100% need because you can do what the guide says in any encounter, but highly recomended for maximum efficiency)
I'd argue that as mentioned above there are better places to find wild Magikarps. The earliest place in the game is Viridian City. Curiously, Magikarp has a Tier 0 spawn here: during Morning and Day it is the only pokemon that spawns when surfing there. Thus, you have around 2:30-3 approximate hours to hunt for Magikarps while no other pokemon spawns. This in my opinion is the best PRO spawn for it as it allows you (outside of the night time) to hunt it as a newbie with just 5 badges. You can purchase a Natu Synch in Trade Chat that has been caught in Route 28 as it is considered a Kanto pokemon. You can also headbutt for it throughout Kanto and pray to be lucky (or hunt Abra and Kadabra synchs with an Arena Trap Diglett lead in Route 25).
Aside from this, the best mid-lategame spawn is obviously Sootopolis surfing as Magikarp and Gyarados are Tier 0 pokemon there. It's very annoying to get there though, we can just circumvent it through Teleportation Quest. Sootopolis itself requires no sidequests.
As a small extra addition: If a newbie is reading the guide, I'd highly suggest capturing a wild Farfetch'd (or multiple) in Route 13 and leveling them up in that same route against Grass Types and Bug Types, then in Route 15 until level 45. It has an insane level-up moveset and it learns False Swipe at level 45, making it an insanely good and early False Swiper in your PRO adventure.
It was a good quick guide 🙂 keep it up!