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I finished a mono Water run on Yellow. I got Magikarp first and grinded him to 20 to beat Misty and Surge, then I got Squirtle, and I did half the game with them until I could get passed Snorlax to get the new rods (it's so stupid how they're set) and caught everything else. Didn't want to level them up so I just put them in my team with the exp share and did the whole game until Cinnabar using only Gyarados and Blastoise.
I leveled the rest up in the mansion and finished the game with them.
Right now I'm filling my dex for the first time. I'm at 118/142 currently.
I might do a mono poison on Red or Blue to get missing mons from Yellow.