Having run some Ultraman inspired stuff during playtesting, I think I'd just keep it simple and use the Scale rules. Mundane is when you're a normal lil fella and then I'd just jump straight to giant for when you're a big ol' fella.
But... if you wanted to get spicy with it? I'd have everyone make one character with the normal rules, but then use the rules for creating a Kaiju to create their "Great Hero" form. Then I'd fight them with Kaiju. I like this approach because it leaves some fun options for military intervention so you can run asymmetrical combats that feature human reaction to giants fighting.
Hope this helps!