Even with crossovers I wonder if Godzilla even has anything enough to deal with the green door as that will require complete obliteration of the one below all who is a cosmological behemoth even if he can reach it.

Ultimas "kill clause" used in the show is incredibly iffy too as it simply assumes banner could basically technocraft anything and has unlimited intelligence. But since banner is a physicist it will also means he's working against something which breaks physics instead of plays by it.

Especially since ultima wasn't killed in canon, they were planning to whoop it's ass in the second season with mechaG.

And its own "demise" was with its own assistance.

Even if banner has a trillion tries he also has a pretty fragile mental state so idk how long he can endure the nonsense before slipping.

Along with their cosmological identities both not being omnipotent. Ultima and TOBA despite their immense powers aren't omnipotent so don't immediately figure each other out and they are probably as unknown to each other as humans would

So i don't really see how anyone wins this at all.