Before anything starts, I'm gonna outline a couple things about Gunvolt and Prevasion, since every time he's in a VS Match (such as against Diavolo) everyone misunderstands his abilities and downplay him, so here I go.
Prevasion is an automatic intangibility defense that turns GV into electrons. It costs part of a resource called EP, the same resource as his Flashfield each time its used to evade an attack, hence "Pre"vasion. By default, he's practically unhittable. As for EP, that resource is rechargable, either on its own, or manually by Gunvolt himself. If he chooses the latter, all of his EP is instantly restored to its absolute max. Combine this with Prevasion, and Gunvolt can stall out anyone if he really wanted to. In character however, Gunvolt does not rely on Prevasion and actively tries to dodge attacks, but would also want to make sure that his EP is topped. Prevasion is disabled when Gunvolt uses his Flashfield, his main attack.
When he is out of EP, he goes into Overheat state, for a second or two, he becomes vulnerable and cannot use his Flashfield or Prevasion, making him vulnerable to any normal attack. However, in character, Gunvolt tries to keep his EP at full at all times via instant recharge and Split Second if needed, the both being spammable
From here, we can discuss whether or not GV should have Prevasion, in battle, he effectively fights exactly the same with or without it, always trying to dodge attacks first anyways.
