How to Keep an S+ Score in Special Mission 5 (Boss Rush Stage)

Ever since the 1.2 update for the 3DS version of Gunvolt was released, I've seen a fair amount of people wondering how you can maintain an S+ score in Special Mission 5, the boss rush stage. There's no denying that this can be a pretty tricky thing to figure out, what with the true final boss and all, so I've decided to detail the method I use to ensure that I obtain the most amount of kudos possible in the stage.
As a quick note, this post will presuppose that the reader knows how to avoid most, if not all of the boss attacks. With that said, you can read my strategy below.

The Easy Way to Avoid Viper's Special Skill; Unlimited Invincibility Trick

Ever since I started playing Gunvolt, Viper has been the bane of my existence. He's simply the worst boss in the game, with an unpredictable attack pattern, lightning-fast movements, and--his most defining attribute--his catastrophic special skill, Sunshine Nova (aka Refulgence). 

This skill is incredibly hard to dodge normally. In the first phase of the attack, Viper throws a rain of small fireballs down on you, and the only apparent way to avoid them is to move slowly through the small gaps in the attack (and pray that you don't activate your command-dash). This strategy requires very pixel-perfect timing, however, and it's very difficult to get down.
I've always sought an easier way to get around this attack, as, to me, it just seems like the cheapest move in the entire game.
Welp, as of today, I've officially found an equally cheap move of my own to counter it.