Wild battles can be a great way to gain levels for pokemon. But, always be aware that with every encounter theres a chance for a shiny pokemon to appear so ALWAYS be mindful in what area you are hunting and take an appropriate sync with you just in case. The sync can be fainted and it will still work. It only needs to be in the first slot of your party. More info on syncs can be found here
Pre E4
Kanto:
The best places to level in Kanto before the elite 4 will be Seafoam Islands and Victory road and make sure you haven’t skipped any trainer battles. The trainers on routes 12-15 and 16-18 and 19-21 are easily overlooked sometimes. Trainer battles are good experience for trainer and Pokémon levels as well as pokedollars so DON’T SKIP THEM!
Johto:
The best places to level in Johto before the elite 4 will be Dragons Den, Victory road and Safari Zone and make sure you haven’t skipped any trainer battles. The trainers on routes 40-41 and 47-48 and 45-46 are easily overlooked sometimes. Trainer battles are good experience for trainer and Pokémon levels as well as pokedollars so DON’T SKIP THEM!
After E4
After the Elite 4 in Johto your best place will be Mt Silver. For Kanto you have 2 options which are Mt Silver and Cerulean Cave.
After B/W 2 story
After the e4 you will be able to make your way to Black City and White Forest which are to the right of Nimbasa city. Above White Forest is route u14 and the dark grass in this area offers 60+ level wild pokemon. The dark grass below the bridge in Village Bridge in Unova also offers high level wild pokemon to grind on.