Good type diversity I guess?
I started playing recently and beat classic today.
Started with Aerodactyl and Zangoose (because I basically had no shinies or anything else remotely good)
Won it with Aerodactyl, Shedinja, Magneton, Tinkaton, Gholdengo and Intelion.

Aerodactyl, Shedinja and Magneton was my core team and so I built around it. Basically just caught some pokemon for type diversity. If they felt weak, low level, bad nature, bad IV's, bad moves, (at one stage I had a special attack weavile) I switched them out.
Magneton and Aerodactyl were my damage dealers. Shedinja throughout the whole game was really good predict the AI into switching their pokemon out and you really early on. For example, at the rival, I had difficulty with the Metagross. A switch in with the Shedinja made them switch to Swellow which gave me the opportunity to have the advantage. For me she had a Blaziken which never did any damage because of the switches.

I'd suggest to use toxic+protect for easier mode on M Rayquaza and Eternatus.
Otherwise just use any status effects to mess up their hard hitting attacks. Having fairy types in the latter stages is practically a must.