Has anyone tried to solo Gengar with a bunch of level 40 Vaporeons when it is rainy?
Yes, it is definitely possible, according to GoBattleSim, about 80% chance to win.
https://pokemongo.gamepress.gg/gobattlesim?%7B"atkrSettings"%3A%5B%7B"party_list"%3A%5B%7B"pokemon_list"%3A%5B%7B"species"%3A"vaporeon"%2C"defiv"%3A"14"%2C"fmove"%3A"Water%20Gun"%2C"cmove"%3A"Hydro%20Pump"%2C"dodge"%3A"1"%7D%5D%7D%5D%7D%5D%2C"dfdrSettings"%3A%7B"species"%3A"gengar"%2C"fmove"%3A"Hex"%2C"cmove"%3A"Focus%20Blast"%2C"raid_tier"%3A3%7D%2C"generalSettings"%3A%7B"weather"%3A"RAIN"%2C"simPerConfig"%3A50%2C"reportType"%3A"avrg"%7D%7D
One thing to note it is only doable by 15 attack IV ones.
Answers
Because there is a fairly high chance if the player was level 38-40 BEFORE gym rework they might have 6 High Level Vaperons.
Vape was the best water attacker in Gen1 (Counting Gary as a Gen2 water attacker because WF wasn't a Gen1 move)
And Vape was what 5th highest CP back then? Meaning in old CP driven towers, it was quite possible to see level 40 Vaperons. Same was true even after Gen2 came out where it was 7th.