I don't see how a shader could do this. I'm pretty sure it's not possible.
Use a canvas instead.
-create a full layout canvas, shouldn't impact performance at all in a fighting game with a tiny layout.
-create a full screen object with erase, set it to 20% or so
-always paste the erase object into the canvas so it is constantly fading away whatever has been pasted into it.
intermittently paste the sprite when the effect is needed, the opacity of the sprite should be adjusted to 50% right before the paste, and readjusted to normal right after(the opacity tweak may not work if the paste action waits until the end of the frame, but i'm pretty sure it will work.).