A Fluxation has a chance of happening when you use a Momentum attack with a Tech (the circle to the right of the player name. When it has a white dot above it, you can press
just as the move starts to add Momentum). There are some Talismans that improve chance of Fluxation.
This is easiest to get in a boss fight as they will take longer. Just keep using Momentums with techs and hope you get 4 Fluxations.