Raiden V: Director's Cut (EP)

This can only be done on the 2 Bonus missions but I recommend doing it in Bonus mission 1 but you need to get all 20 in one go. On top of the screen, there is a bar that you need to fill and to do so you need to collect medals. The boss has 3 phases and if you fail to collect enough medals for next phase, the bonus mission will end. To the right off the screen, it shows how many medals you collected. They are divided in Gold, Silver and Bronze. You can't die on the bonus missions, however if you get hit then you will lose medals so don't get hit too many times.

In order to get 20 Gold, you need to play aggressively and use Cheer attack and your bombs. To make the medals come to you, simply stop shooting for a second but the best weapon for this is the Swing Vulcan. See the following video for a demonstration: