Try smashing the planet with a moon very tiny compared to the Earth's moon but larger than the asteroid that killed the dinosaurs. I know that's not what you want but your victims will be just as dead.
Suppose that your target planet has a very small moon orbiting at a great though realistic distance. Suppose that the people of the target planet don't have many facilities on that small moon or consider it worth defending.
Possibly they have a lot of important facilities on closer moons and orbital space stations a lot closer to their planet. And they might have super force fields around their entire planet and those close orbital facilities to defend against any attack. And giant orbital ray guns to blast any space ship that comes close to the planet and its orbital facilities.
So the attacking space navy comes and blasts at the planetary defense shields at a safe range and the planetary defense ray guns keep them covered, firing whenever they get too close. And many ships duck behind the outer moon and pop out from behind to fire at the planet and then duck back into cover. And the planetary defenders think that's all they're doing.
But some of the space ships are unloading parts for a giant space drive, a giant rocket or anti gravity engine or whatever. It took them months or years to design and build an engine so much bigger than any built before. And using tractor beams they are assembling it on the back side of the moon but pointed forward so that its trust will slow down the orbital speed of the moon.
So they turn on the giant engine and it quickly slows the moon's orbit so that it soon has no speed relative to the planet and falls toward the planet under the gravity of the planet.
It will take just a few days to smash into the planet, just as Apollo space craft took about three Earth days to "fall" from Lunar orbit to low Earth orbit.
The defenders try blasting it with their giant space ray guns, but they can't vaporize away enough of the moon's mass in a few days. It will still have enough mass and velocity to cause an extinction level impact.
They can't build a giant space engine to take to the moon and change its velocity and trajectory in the time they have left.
They send ships with giant atomic bombs to try to reduce the mass and/or change the velocity of the falling moon, but those are all blasted by the attacking space armada once they pass outside the defense force fields.