The setup shown here involves using Jump to unmorph, then holding it for the duration of the damage boost. The power bomb is laid near the top of the spike in the background texture, so that it's low enough to open the door, while not being so low that it destroys the Viola.

There is a faster setup that involves planting the Power Bomb directly next to the door, and then damage boosting off of the Viola before it gets killed by the blast.