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.
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). pokemon fire red 100 save
Unlike later games, FireRed has no roaming dogs (those are post-National Dex in FRLG via events only). That’s good. But the event-locked Pokémon (Mew, Deoxys, Lugia, Ho-Oh) are impossible without cheats or old distribution cartridges. A true 100% is technically unattainable legitimately today. This is the single hardest challenge