So: how do you create the big, almost unstoppable thing, and have it true to the superhero genre?
The history of the Unstoppable Thing in superheroes is that it can be stopped but not by direct force. Instead, you stop it by some other means, whether it's talking to Galactus, or coming up with clever gadgets that allow you to literally keep your promise not to set foot in Gotham City for one week...and still fight crime!
So that's one way: use its Qualities against it. Still, if you do that every time, it's not effective. The players start to go, "Oh, this week it's a Lovecraftian monster. How do we out-think it?" If that's your go-to for "interesting" games, then maybe you should be playing something else. While that sort of thing makes a nice change, the players are often playing a superhero game in order to do great things. Futility they can get talking to the tax people.
So you have a kaiju and it's meant to be a big threat, but not necessarily an impossible one. Things you can do--
Big NumbersThe ICONS system works pretty well if you keep adding numbers past 10. It's not ideal, but I haven't actually found the rule that says that if you have ability 10, you can't have a specialty that works with that ability. (I'm not saying the rule doesn't exist, just that I haven't spotted it.) The mechanics work if you're Strength 3, 10, 13, or 20.
You do have to keep in mind that whenever there's a difference of more than 5, the dice can't help you. The characters simply cannot succeed at hitting the Big Thing if it's Coordination 19.
Furthermore, the PCs cannot damage something with Damage Resistance 10. Oh, maybe they can push or you've cleverly given the Big Thing a Quality that they can activate and use to hurt it, but they can't simply whack at it.
Big numbers are really useful if what you want is something with a huge Stamina but you have to make sure not to give it impossible defenses, too. I don't know about anyone else, but I get discouraged if I have to hit something 2,000 times in order to generate 25 damage and knock it over. It's just grinding away, and I think that the Army's howitzers are better at that kind of thing.
Pyramid TestsSomething that is supported in the rules is generating the hare-brained scheme that might stop this thing and representing it as a pyramid test. They have to get a certain number of successes before they get a different number of failures in order to beat the Big Thing.
Something that I prefer is letting the players come up with the scheme and then re-casting it in terms of a pyramid test. That lets them come up with the brilliant idea (and it often is a brilliant idea) and just using the mechanic on that idea, rather than starting by saying, "Oh, they can run a wide pyramid test to irritate its eyes and thus blind it!"
You might also break it down into a series of pyramid tests. Then if they fail at any particular part of the scheme to defeat the Big Thing, they can come up with an alternative.
You have to be careful, though: you need to walk the line between "yeah, you fail until I say you succeed" and "it's a couple of dice rolls, no biggie." Success isn't pre-determined (if it is, don't make it a roll).
Hit LocationsYou could treat the Big Thing as a set of smaller monsters or hit locations. (This is the inverse of treating a swarm as a single individual.) This works particularly well if one part is less well defended. The players have to take out the legs but the radioactive breath might still get them. Again, individual parts succumb to their attacks, and when they get all of them, the Big Thing is defeated. This works well for kaiju but less well if they're facing a Galactus-level threat.
Invent a Scale
Still, you have this threat...how do they affect something off the scale? This is the superhero genre: we assume that they can achieve what they need to, somehow. But how?
- Claim that anything that's level 10 can affect something on the next scale.
- Attacks inflicted using by invoking a Quality of the Big Thing automatically do damage. ("Hmmm...who knew it was affected by marshmallow chicks?") Maybe an attack using a Quality only does 1 stamina damage, maybe more; I think that would depend on the situation.
- Advantages of a different kind--this is just talking to it and finding its Qualities--the Galactus method.
- Put the characters on that scale...this is more of the "Dr. McGuffin invented a Cosmic Enlarging Machine: go and kick that kaiju's butt!" but it could be good goofy fun for a night.