Multi-stage missiles are an alternate/great way to fire bombs longer distances. In the example, you can see it consists of creating stages of ballast (or other blocks) that have downward facing cannons (or shrapnel cannons in example) to propel the missile upward, and then decouplers the fire simultaneously to each next stage firing. A grabber is used to mount the bomb to the top.
In example video, due to the abnormally long distance, all missiles were braced together & small wings were added to stabilize and add additional mounts for cannons and propellers to supplement thrust.
In multi-stage missiles, aiming can be accomplished by removing cannons from the side of the direction you want to go. You should experiment to see which how many and which stage(s) are best to remove the cannons from. In example video, a protective wall was added which created a vertical that the missiles had to be propelled over before they could move forwards to the target. Thus, canons were removed from the 2nd level instead of the 1st. This allowed the missiles to be propelled up for a stage to pass over the battery (barricade), the second stage caused the back to propel more quickly than the front (due to the removal of front cannons second stage), then the final to stages (with full cannon capacity) to propel the missiles on that diagonal trajectory, accomplishing full net energy potential.