Add support for multi battles with a singular wild opponent

[pkmnsnfrn]

        setwildbattle(SPECIES_EXCADRILL,100,ITEM_MUSCLE_BAND)
        playmoncry(SPECIES_EXCADRILL,CRY_MODE_ENCOUNTER)
        msgbox("roar",MSGBOX_AUTOCLOSE)
        delay(40)
        waitmoncry
        multi_fixed_wild(TRAINER_WALLY_VR_2,TRAINER_BACK_PIC_WALLY)

I'm trying to create a boss battle where the player and a rival battle against a very strong wild Pokemon.

When I only pass one argument to setwildbattle, I get MissingNo and the Excadrill. I've been told this is a limitation of the current state of affairs and would require a non-zero amount of work to support.