where do I have to put this save?
On PC, there should be a shortcut to your Save folder (conveniently labeled 'Save').
On Mobile, your save is in the main folder.
In either case, it's named Game.rxdata.
Fairly sure "normal pokemon length gameplay with actual pokemon fights" is the main draw of this game, so the chances of that change are slim.
I certainly wouldn't want to cut Trainers*.
*To fuck or be fucked by + a source of Eggs for Pokémon you can't get from Wild encounters.
I actually don't think it's the number of fights, it's the level curve, especially toward the later parts of the game. The (2) Wintercress Trainers are level 36, Ivan is 42. The enemies in Sylph Co are 46 and the bosses are 52. Mt. Granite is level 30-44. Emmie is level 44, so she's not going to be carrying you. And Mightyena's Moxie can trigger off of her Pokémon.
The best ways to grind are vs Jamie's rematch, Forrester, Diglett Man and Alphas because they scale to your level.
Rematches for all Gym Leaders are planned, so that should also help with grinding. Routes aren't fully populated yet, either.