This is the single hardest challenge. One mistake (a crit, a miss, a bad switch) resets you to 0. And you have to do it in each of the 4 modes. It’s not fun – it’s a test of patience and RNG. The final reward (a Silver/Gold shield) is cosmetic only.
Without the VS Seeker, leveling 386 Pokémon would be soul-crushing. Re-battling the 6 trainers on One Island’s hot spring path (Golducks, Gravelers) is the most efficient EXP in the game. It’s slow, but reliable. pokemon fire red 100 save
The Berry Forest on Island 4 and the daily respawning berries add a light farming loop. For 100%, you’ll want rare berries like Lum and Micle for contests (which you also need to master if you transfer to RSE, though not strictly required in FRLG). This is the single hardest challenge