Note: This will likely be the hardest category to learn, the speedrun hard requires RNG manipulating Mudkip and Abra and the Abra especially can be hard to learn.

Follow these notes roughly:
https://pastebin.com/Prv73bGb
Amendments: skip wingull manip and instead mud slap until you have Water Gun on the first Roxanne gym fight. Enter the Mauville center before the Mauville gym fights after getting the bike. After late herb shop, don't do desert candy and instead Teleport with Abra back to Mauville, bike down and take Briney's ship to Dewford. Enter the center in Dewford and take Briney's ship back to Petalburg. After defeating the Petalburg gym, Teleport and you'll be trapped in Dewford!

Example video
https://www.youtube.com/watch?v=TQ7tnN2ik14