Depends on how much prep time they, I don’t know if the maulers can clone the kaiju as they would have to take its dna and then make a body which would be massive and take time(Also would need a machine that big).

1st and best plan is the frequency. Take over every sound system on the planet and play that frequency. Also pray to god that they don’t have any technology that protects their ears from the sound.

Second plan which requires luck. For this you need to once again pray and hope angstrom levy shows up and he agrees to help you, get him to grab Viltrumite killing weapons from across the multiverse.

This is all assuming the Viltrumites once they figure they can’t win, they just destroy the planet. Grab some nukes and launch them at the most populated cities.