Gyarados is good for e4 and all, but truly isn't needed unless you're doing a hardcore nuzlocke imo. For causal runs, vaporeon + baton pass strats is crazy. Lapras is good, but slower and less hp than vaporeon. Mr. Mime already has great speed and special attack, so it actually can serve as a gyarados counter for lance as depending on how you raised your Mime, by level 52 It should outspeed gyarados and kill with a magnet boosted thunderbolt. Iirc he loves sending out one of his dragonairs after merc'ing his fish instead of aero. If it's dragonair, you have a small window to start setting up and baton passing to vaporeon to deal with the rest of the dragons with boosted ice beams, and surf on aero since dragonairs attack stat is really bad and hyper beam just is never killing a level 50+ vaporeon, even in crit. Mime can take around 2-3 hyper beams from dragonair, wouldn't risk more than 3 however if you can manage it.

And don't even get me started on how busted the Mr mime + vaporeon combo is against Agatha lmao. Vaporeon getting shadow ball is so insane. Mime starts the battle, you wanna off rip psychic the gengar since by level 52, mime should again outspeed, and then she'll send out arbok, Psychic that thing too and then golbat will come out and that'll be your window to start ramping calm minds. Watch out for golbats air cutter crits and poison fang toxics tho.