So with no exp share being in this game I've got just a few questions

 

Do most people complete the main playthrough using one Pokemon? (You'd be surprised how much people actually do this) or train a whole party or maybe 1-3 solid pokemon?

 

Also can trainers be battled repeatedly? Or would most of the grinding have to be done through wild pokemon?

I've trained a full party, to lvl 85.

 

Yeah, you can rebattle trainers after 7 days (except gym leaders).

i did the main play trough with 5 pokemon which is doable. at e4 i had 2-3 main pokemon and the others where revive/pot sacrifices.

I finished kanto story with 5 Pokemon. For jotho I had ms and exp-boost and just rushed it with my cyndaquill which was lvl 97 as i reached the 8th badge. Reading about how hard grinding in jotho is, I think I did everything right cause I had not a single moment of struggle throughout jotho until last badge.