Did you do it? That maneuvre would take decades to millenia, and when you use something that far, it may be more efficient to push it into a retrograde orbit so that it hits Earth heads on, instead of chasing after it. It's just absurdly impractical.
Anyway, if you really want to destroy a planet, you want sonething small but fast. It penetrates into the planet, and rips surface on the other side.
