Ok. I've worked stuff out to the point where I'm happy with it. Thanks all.

It turns out that hunting/sniper rifles, and even more so laser sniper rifles, are pretty much the best bet for anything past the most basic troops. After that its the anti-armor weapons (ap grenades, ap rockets, short range plasma blasters). Even with enough good weapons the monsters will still kill 200 to 500 regular troops, half that in veterans with heavy armor, and half again elites in power armor. But I have my numbers and they work on both the narrative & mechanical levels now.

One issue I hit was that the system in use is set up so once a critter runs out of hit points/plot points it gets into critical damage, which is what does the actual killing with bleeding, KO, stuns, dismemberment, eviceration, etc. There are abilities creatures can have to mitigate or be immune to some of this stuff (eg: robots don't bleed or suffer fatigue), or there's one tag that says "ignore critical hits that wouldn't outright kill the creature unless <spacial conditions>". Playing with the simulator that ultra no-crit tag makes it super hard to kill the monster if it has otherwise lore appropriate stats. Without that tag, but with all the other immunities turned on, ten regular soldiers with gyrojet pistols they aren't proficient in can kill the monster in about 5 rounds. It got better as I upped the monster's stats but to make it sufficently resistant in order to get even closr to the correct narrative results would take boosting its stats to insane levels.

Obviously I could hand wave & butt pull or just make up a new monster ability. But that affects playability. If every big monster has unique & totally different defense abilities then players have no rules consistency or planning ability aside from "do big damage" (boring). If the monster is under statted relative to the narrative effects players get nasty cognitive dissonance when they mow it down using less damage abilities than a couple squads of mook npcs. If its over statted the players basically can't engage with it, having to give up & abandon ship every time it shows up or it's super tough but has weenie nerf claws that can't do the required damage to be a real threat.

Again, thanks for the help. And now I know they won't try something like the powder magazine idea.