"Wah this better work Eggman-"
"Oh come on Waluigi since when do my plants never work?"
Waluigi was quick to cast a quick glare at Dr. Robotnik which was enough to make him shrink in losing confidence; alas the 'evil Luigi' was used to people like Eggman being so full of their egos and yet it wasn't of any concern so long as the plant went accordingly...
...which is why they were building rockets that they launched: and all of them happened to 'accidentally' hit a particular target: the Smash Mansion. And luckily for both of them they were successful!