One thing I do with stuff this big is treat the creature more as an environmental hazard than a "creature." So, lots of saving throws and such.

I tend to think human-sized heroes, regardless of their individual power levels, are not going to really be able to kill something like a kaiju. All you can do is drive it off, or distract it from its goal, or something along those lines. I wouldn't give Godzilla hit points, for example. Fighting him is like a mini-dungeon -- you do different things, make progressive changes to his goals and attention, and eventually "succeed" by getting him to dive back into the ocean or whatever makes sense in context.