Spike planting
Generally, if you can, try to plant the spike exposed to multiple angles so it's easier to shoot the defuser. However, on some sites the optimal plant spot is in a corner that's exposed to fewer angles because in the corner reduces the defusable area and makes it easier to spam the defuser.
If you are smoked off or Sova ulting the bomb, ping it on the megamap (or have a teammate ping it). You can see the ping through everything and know where to aim.
Shooting
The walls are made of paper. Very often, spamming a wall will actually get bullets through, and might net you a kill.