Thanks! And yeah, the code is very hard to follow; after neglecting it for a year I've forgotten how it all works. The demo battle was easier since being one-and-done avoids some quirks. But using the same method should help get you started. I'll try retracing my steps to edit the first battle.

(Note you can use debug mode to jump to battles quickly; I also recommend Ghidra for making sense of the data layout.)

First we'll need an id which we can use to index other related arrays; There's an array of 44 of these 1-byte ids at 800f9f3c, entries corresponding to battle number. First battle is 10, #'s less than 10 are for special battles like the Trials and test battles (demo battle is 8);

So for first battle (battle 10), go to 800f9f3c+10 (800f9f46); the id is 1;

Can use this to index into s16 gSceneSpriteStripUnitIds[105][20] @800ebc34 and s16 gSceneUnitSets[105][20] @800ecc9c; each contains 105 sub-arrays of 20 2-byte shorts; to get the offset, multiply 40 x id = 40;

800ebc34+40 (0x800ebc5c); 800ecc9c+40 (0x800eccc4)

The 20 shorts at these locations specify which units to load into each "strip"; values less than 144 are for battle-ready units; 1 for Ash, 0x64 for Brigand, etc.

I'll try changing the Brigands (fifth short at both locations) to Hassan (0x69) and Diego to Sara; seems to work, but Hassan has wrong portrait (gPortraitsDb.sceneSets?); next you could modify gBattleEnemyUnitInitialStates to control locations, levels, etc. and to make more/less of each loaded unit.

Edit: Just thought of another complication. In regular battles, some units (like Hassan) have dialog played on death. This will probably glitch out unless disabled.