I've done some research on set wild battles in Emerald, and it turns out to be more complicated than what I anticipated. I can't guarantee that this is the same for Firered or any other ROM.
First of all, the command "setwildbattle PKMN LEVEL ITEM" is the same as doing "setvar 0x8004 PKMN, setvar 0x8005 LEVEL, setvar 0x8006 ITEM". Also, the flag 0x8C1 gets set before every battle and is cleared afterwards. I'm not sure why this happens. I don't see the difference when I don't include this.
A set wild battle is always followed by a special that determines the battle type, which is 0x139, 0x13A, 0x13B or 0x145.
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SPECIAL 0x139
Special 0x139 doesn't seem to be used anywhere in the game, but it has a wave-like intro with the Kyogre/Groudon/Rayquaza battle music. Let's call it the "Weather Trio Song".
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SPECIAL 0x13A
The special 0x13A is the strangest one, because it behaves differently depending on which Pokémon you're battling. This special is used on the Groudon, Kyogre, Rayquaza, Deoxys, Mew, Ho-oh and Lugia events.
If this special is used on any other Pokémon than the ones I mentioned, it will have the same intro that Groudon uses. Groudon image, cave background and the weather trio music.
When the special is used on Kyogre, it will have the same music but with a slightly different image and water background in battle.
On Rayquaza, it has the same music, different image and a sky background in battle.
Ho-Oh and Lugia have the same type of intro here. The screen fades black three times, then it pixelates. It uses the Kanto-legendary battle music.
Deoxys has almost the same as Ho-Oh and Lugia, except it uses a battle music that is exclusive to Deoxys.
Mew's intro fades black three times, then has a checkered-type of animation before going into battle. Not sure how to explain it, but it looks to be the same intro in special 0x13B. Mew also has its own exclusive battle music, which is similar to the Kanto-legendary music.
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SPECIAL 0x13B
This special is only used for the Regis, and the intro depends on which of the three Pokémon is set in the wild battle.
First of all, if you use a Pokémon that isn't a Regi, it will have the Regi battle music but it will have the same type of intro that Mew uses.
In the intros, Regice shows seven yellow circles on the screen, Regirock shows seven orange circles and Registeel shows seven red circles. All in different patterns.
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SPECIAL 0x145
This special does not behave differently depending on the Pokémon, but on the level. If the Pokémon is the same level or above your first Pokémon in the party, the intro will show white stripes. If it's under your Pokémon's level, it will be a completely normal intro, like a wild encounter in a grass patch. As far as I'm concerned, this special is pretty much the same as using a "wildbattle" command.
In the game, this special is only used on Southern Island for the Latios/Latias that isn't roaming, at level 50.
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Here are videos with the scripts in the descriptions:
https://drive.google.com/folderview?id=0B1n0SW2yBMufV18tNTdITkxhaEk&usp=sharing
I'm not sure how to change these battle types that are set to determined Pokémon. It might be possible with some ASM, but I have zero knowledge about that.
However, it's pretty easy to change the battle music. Just add a "playsong" command and use any song you like.