LA Overworld Legend Calculator.cs

// Global settings to make things easier.

int FlawlessIVs = 3;

// Set these values for your gender ratio:
//    Fixed gender = -1
//     50 M : 50 F = 127
//     25 M : 75 F = 191
//     75 M : 25 F = 63
// 87.5 M : 12.5 F = 31
// Litleo line doesn't exist in LA so we can ignore the last case.
int GenderCompare = -1;

// Set this for how far you want to search.
int maxAdvance = 100;

string output = "";

void Main()
{
    // Initial state
    ulong group_seed = 0x286e5c32787a9d20u;  // group seed
    ulong init = unchecked(group_seed);
    Xoroshiro128Plus rng = new(init);

    output = $"Initial Group Seed: {group_seed:x16}\n";
    Console.WriteLine(output);
    Console.WriteLine("Adv | Generator Seed   | Pokemon Seed     | PID      | IVs               | G | Nature\n----+------------------+------------------+----------+-------------------+---+--------");

    for (int advances = 0; advances < maxAdvance; advances++)
    {
        output = $"{advances, 3} | ";
        GenerateSpawn(advances, ref rng);
    }
}

private void GenerateSpawn(int advance, ref Xoroshiro128Plus rng)
{
    // Group seed generates the generator seed and alphamove seed.
    var genseed = rng.Next();
    output += $"{genseed:X16} | ";

    var alphamoveseed = rng.Next();

    // Generator seed generates the slot, mon seed, and level.
    var slotrng = new Xoroshiro128Plus(genseed);
    var slot = slotrng.Next();

    var seed = slotrng.Next();
    output += $"{seed:X16} | ";

    GeneratePoke(seed);

    // Reseed the RNG for the next genie.
    var newseed2 = rng.Next();
    rng = new Xoroshiro128Plus(newseed2);
}

private void GeneratePoke(ulong seed)
{
    var rng = new Xoroshiro128Plus(seed);

    rng.NextInt();
    rng.NextInt();
    var pid = rng.NextInt();
    output += $"{pid:x8}";

    Span<int> ivs = stackalloc[] { -1, -1, -1, -1, -1, -1 };
    const int MAX = 31;
    for (int i = 0; i < FlawlessIVs; i++)
    {
        int index;
        do { index = (int)rng.NextInt(6); }
        while (ivs[index] != -1);

        ivs[index] = MAX;
    }

    var ivstring = "";
    for (int i = 0; i < ivs.Length; i++)
    {
        if (ivs[i] == -1)
            ivs[i] = (int)rng.NextInt(32);
        ivstring += $"{ivs[i], 2}";
        if (i < 5)
            ivstring += "/";
    }
    output += $" | {ivstring}";

    rng.NextInt(2);

    var genderchar = "-";
    if (GenderCompare != -1)
    {
        var genderval = rng.NextInt(252) + 1;
        if ((int)genderval < GenderCompare)
            genderchar = "F";
        else
            genderchar = "M";
    }
    output += $" | {genderchar}";
    
    var nature = (int)rng.NextInt(25);
    output += $" | {GameInfo.GetStrings(1).Natures[nature]}";

    // Add your filters here.
    //if (nature != (int)Nature.Timid)
    //    return;
    //if (genderchar != "M")
    //    return;
    //if (ivs[0] != 31 || ivs[1] != 0 || ivs[2] != 31 || ivs[3] != 31 || ivs[4] != 31 || ivs[5] != 31)
    //    return;

    Console.WriteLine(output);
}

This script shows future advancements for overworld legendaries in LA such as Heatran and Enamorus for RNG manipulation when given a group seed. Use this script to solve 2 Pokémon for a group seed.

Setup is:

1. Download/install LINQPad 7 from https://www.linqpad.net/LINQPad7.aspx
2. Download dev build PKHeX from https://projectpokemon.org/home/files/file/2445-pkhex-development-build/
3. Press F4 in LINQPad and add PKHeX.Core.dll to first tab, and PKHeX.Core to second tab,
4. Click Edit > Preferences, select the Results tab, click "Launch Editor" and paste this in to get monospaced text.

body
{
	margin: 0.3em 0.3em 0.4em 0.4em;
	font-family: Consolas;
}

5. Change the language at the top to C# program, then copy/paste the script in verbatim.
6. Edit your params (thresholds, advances, gender).
7. Click run.

This is not meant to be a polished or complete tool in itself, nor are there plans to make an easier tool with a GUI.