A simple guide on the One Badge category using the Tentacool/Blue route. I'll be splitting this up into 3 parts, Intro, Tentacool, and Blue.

Intro: Essentially, it's mostly holding B (to get through the text ASAP) and mashing A to just get through the menus as fast as possible. Note that it is faster to hold RIGHT rather than to mash RIGHT when going through the character selection and starter selection. You will use Seel for your starter since it has Surf. Remember that once you see Slowpoke, be ready to press A on the next Pokemon, which will be Seel. After mashing A (and holding B ) for the 3 time select boxes, switch to mashing B (and holding A) for the rest of the textboxes to avoid going through Shock's intro text/saving/setting a nickname on Seel. Once you gain control of your character, press Start, go UP 2 times, press A, press LEFT 2 times (for setting Text Speed to NONE), press DOWN, press LEFT (for setting Battle Animations to OFF), press DOWN, press LEFT (for setting Ask Switch to NO), press UP 3 times (to get to the next page of options), press A, press LEFT (for setting Encounter to FAST), mash B (for getting out of the menus), and that should set the options as fast as possible (under 5 seconds if you are quick).

Tentacool: Now that everything is set up, you want to hold B to run to the water near Pallet Town. However, the water in Pallet Town won't work, you near to go a little further to Route 21 (marked by the green ground). Once you get on Route 21, turn LEFT, mash A to Surf on the water, spin around (hard at first but you get better with practice), or move LEFT/RIGHT or UP/DOWN if you don't mind the slower method. Soon you'll get a Tentacool or Tentacruel encounter. Once you encounter Tentacool, yolo-ball it, it's about a 1/3 catch rate so keep trying the run until Tentacool stays in the ball. Once you get near perfect execution, getting a fast Tentacool will pretty much decide how fast a run is. To note, it is generally better for the Tentacool/cruel to be at least level 36, as it will have Acid as its first move and it will pretty much be guaranteed to OHKO all of Blue's team. Once you get Tentacool to stay in the ball, mash B until you get into the overworld. Now press START (to pause), press DOWN (to select POKEMON), press A, press DOWN (to select Tentacool), press A, press DOWN (to select SWITCH), press A, press UP, press A. This will swap Tentacool to the top of the party. Afterwards, mash B to get out of the menus.

Blue: Basically, hold B and run through Route 1 and hope the one spinner doesn't spot you and you don't get any wild encounters. If RNG is nice, you won't get any. Get to the Viridian Gym, talk to Blue, mash A to get through the text boxes. If Tentacool is level 36 or more, just mash A throughout the battle, since your first move will already be Acid. If Tentacool is less than 36, then press DOWN once when selecting the first move to highlight Acid. Generally at this point you just mash B and time the run after Blue goes through all his dialogue.