After the two previous fights you’d think Energy Flux would be a nightmare, but it really isn’t. This fight is all about playing smart and paying attention to your surroundings.
For every fight I strongly suggest using the gauntlets. Not only are they fast and deal decent damage, the charged punch attack has invincibility frames that make players immune to energy based attacks. From there, no player should need potions outside of the last fight and upgrades are helpful, but only important if you want to win without taking damage.
Since you can’t damage Energy Flux before it moves, simply wait in the middle for it to move. At this point you should have enough time to get five or so hits in before you want to retreat and see which attack it does. It won’t take long to figure out which it is, so get ready to counter them.
If Energy Flux moves, simply hang out in the middle and stay out of it’s way. Red lightning can be countered by charged fist attacks, which is also true for blue lightning, though that is better to just dodge roll over. As for it’s final attack, look at where lightning isn’t in front of the boss and stand there. By being smart and watching attacks, you should be able to beat it with relative ease.