Well anyway, I had some thoughts on the OP.
The situation is basically; your big buy is that kaiju can exist without collapsing, issue forth from a portal under the sea, sometimes have special abilities and are resistant to conventional weapons though not impervious. However, unlike the movie building a giant mech is not an option, we have to fight back with our understanding of how the world works.
Seems to me like only the first few would go undetected, if that. After those were bombed/nuked and the accompanying cities lost, we'd have navy and air force on constant patrol along the Pacific and would know about all new creatures prior to their arrival, so that ideally we could fight them while they're in/on the Pacific. Subsequent to that it's a matter of holding them at equilibrium while someone figures out where the hell they're coming from. Assuming they all just head for the nearest population center and all come from one portal, we'd inevitably figure out where it is, at which point the priority would be destroying the portal by whatever means necessary. We could also install very early detection systems at the very least so we'd always know the instant a monster appeared, and over time, presuming the monsters have generalized weaknesses we'd learn what those are and slowly perfect the anti-kaiju machinery.
Nodachi unrelated.