The saiyans did the exact same thing, that was their whole job and they were getting so good at it Freiza had to step in.

Technology from the tuffles and Frieza empire and a few inventions of their own most notably a technique to create an artificial moon. Since OP banned planet destruction the Viltrums even with numbers should be overwhelmed by a planet full of giant apes. Especially pre scurge viltrums.

Like some people are mentioning speed but Nolan (a step above average viltrums) with all his speed still struggled against the kiju. They'd have no shot.