Blast to the Stone Pillar is the fifth mission of Jolly Roger Bay.
Select Star 1 to reduce lag by spawning the sunken ship. Consider your star order though because doing so will make the Jetstream star and the Eel star not spawn. Saving the cave star until after the cannon star will ensure that you have a backup if you miss the cannon shot.
Aim under the V-shaped top of the grey wall texture with the middle of the top crosshair approximately horizontal to where the lowest part of the sky can be seen through the wall. Other reference spots are available; take a look at the other images to find a spot that works for you.
Visualization of all of the possible cannon shots that will work. The green area means that it will work with minimal dust, the yellow area means that you will need a lot of dust, and the red area will not work at all.
Best spot to aim for. Aiming farther left gives you a better angle towards the star.