One method I like is the Black Hole Bomb. I believe it can be done by 17th.
Take 1 part Genesis spell, 2 parts Gate, and as large and as dense of a substance you can acquire (neutronium perhaps?). Cook your Genesis demiplane in a complete vacuum, with ultraheavy gravity in a single direction. Make a portal horizonal with the planet you wish destroyed. Then make two gates, persisted as needed, face to face with as little space in between as possible. To ensure nothing gets in there you might want to perform this either in the depths of space or in a vaccuum sealed chamber (a box of force and removing everything with a gate to the plane of vaccuum works nicely).
What you need to do is drop your dense object through the gate to your demiplane, where it will appear at the top of your demiplane and accelerate from gravity. It exits via a gate at the bottom of your demiplane, only to immediately fall through the top.
Speedy thing goes in, speedy thing comes out. Repeat for endless acceleration in a single direction. Once you have achieved the desired velocity (which since this is acceleration by gravity, I believe can exceed light speed) remove the gate leading to the top of your demiplane. The object will smash into the ground at such speeds that there is a fair chance that you've compressed the material into a black hole. Even if you didn't, you've just shot the planet with a giant rail gun slug and probably blew a hole clean through it. Collect the possibly hyperdense debris and try again.