This Pokemon FireRed and LeafGreen trick will get you practically any Pokemon without any problems

Pokemon FireRed and LeafGreen are notorious for the many bugs they've created over the years, whether it's something that players could use to their advantage or something that was always harmful. There are many examples. One is how Pokemon FireRed and LeafGreens email duplication bug allowed players to dupe any item they wanted. Another is that the roaming Legendary Beasts can break the game and never respawn after using Roar, and while this is fixed in the current Switch version of the games, the other bug about their IVs always being 0 is not. Still, there are ways to take advantage of what the game offers without any trouble.

One such thing is RNG manipulation, which allows you to get exactly the Pokemon you want from a list of possible seeds, provided you do the trick correctly. This is not to be confused with ACE, which instead generates Pokemon out of thin air and even lets you create NPCs to trade with to get trade-exclusive evolutions solo. Pokemon FireRed and LeafGreens RNG manipulation isn't easy though, and you have to be very precise to actually get anything out of it. But if you manage to do that, you will be able to get practically any Pokemon.

Shiny Charizard, Psyduck and Sandslash in Pokemon FireRed and LeafGreen

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Pokemon FireRed and LeafGreen's best shiny hunting strategy is unfortunately exclusive version

Shiny hunting in Pokemon FireRed and LeafGreen is always at full odds, but the best trick you can use is exclusive to one version.

How to Find Your Secret ID in Pokemon FireRed and LeafGreen

The way RNG manipulation works is to use software like PokeFinder to check out the Pokemon you have to determine your SID (Secret ID), which is then used to determine what type of Pokemon you can find, and then use a set of tools to get you those Pokemon. The basis for finding your SID is as follows:

Catch a Pokemon, check its gender, nature and stats and keep track of them.
Upgrade the Pokemon either through Rare Candy or by battling wild Pokemon and trainers, and keep track of any EVs it wins in the process. Write down any additional changes to the Pokemon's stats per level.
After a few levels, use a Gen 3 IV calculator like this one and input all the correct information about the Pokémon, such as species, nature, level, current stats, and any EVs.
Until you limit your Pokemon's IV to a set of specific numbers, repeat the process above. When determining your Pokemon's IV, use PokeFinder, go to Gen 3 Tools and click IV to PID.
Insert the Pokémon's IV along with its character and your trainer ID (found on your trainer card i Pokemon FireRed and LeafGreen). This will determine your SID, which you can then use to find the Pokemon you want.

Drag weapons to fill the grid

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How Pokemon FireRed and LeafGreen's RNG manipulation works

Finding your secret ID is the only way to make RNG manipulation work. After that, the first step is to go to a website called Ten Lines to search for specific Pokemon. You can use filters to choose whether the Pokemon should be shiny, have a specific gender, have a specific character, or have a given range of IVs. There are two main things to keep in mind here:

RNG manipulation does not generate Pokémon, but rather uses existing Pokémon seeds to allow you to catch the exact Pokémon if you time the process correctly.
Seeds are limited and tested by community members, so there may be other options that you don't see.

Then you can use a program called EonTimer to help you determine when you need to press A on the start screen, the save file screen, and finally on your static Pokemon (which Pokemon FireRed and LeafGreen starters, Eevee in Celadon City, etc.). This also works for fishing, via Rock Smash, wild Pokemon via Sweet Scent, event Pokemon and Legendaries. However, you have to perform three perfectly timed button presses in a row, which is not easy. If you miss your goal, however, Ten Lines and EonTimer have ways to help you calibrate the process, which means you enter data to adjust the timers accordingly until you finally reach your goal.

Although this method can be complex and intimidating at first, it allows you to get basically any Pokemon in the game without cheating or using errors, and that includes rare animals like shiny Mewtwo, Legendary Birds, and more. Assuming you find your SID with a random starter, you can reset the game to the moment before choosing a starter if you haven't saved in the meantime, and then proceed with RNG manipulation to get a shiny starter in Pokemon FireRed and LeafGreen without spending dozens of hours in Oak's lab.