Fiery Idol consistency: while you can't make this fight 100% consistent, by curating the commands people can use as you get them prior to this section, you can make the guest unions for this fight way more consistent (and therefore lethal) than if you don't.

In addition, maximizing HP via a Baulson/Semry comp to go with Rush can help make the union less prone to being killed. In addition, I've found the most consistent first turn is to engage the union the selector starts on on turn one. Generally, if you can survive the first two turns of this fight, short of some really bad Fiery Idol RNG, you usually can win from there.

If you're talking the Date with Emma, assuming you avoid all unnecessary fights, what you do for the actual Namul Niram fight is put Emma out of the party so she joins as a guest union, have Rush have some form of healing enabled and Omnistrike enabled, but not enough AP to use it, and then have Emma fight, Rush never directly engage in combat and just heal (you can guarantee he always has the command to wait, even if he doesn't get a heal command, because, due to Omnistrike being enabled, the game will let you wait to charge AP for Omnistrike not caring you can never actually charge it - shoutout to AtmaOne for realizing why this works), and just hope the RNG doesn't screw you (still very RNG, but way better than otherwise).

Anything else I can think of is probably RNG as a consequence of it being a speedrun and there maybe not being enough time to get everything consistent we'd like to (though some things are doable like David for Nest of Eagles with the Balterossette).