The Witcher 3: Wild Hunt - Complete Edition

I had a hard time with the Champion of Champions (the final fight ) and drinking potions or eating food just prior to the fight didn't help ... the combat is so long that those effects wear off before the fight is over.

What I did find that helped is to lock the combat into a pattern. Boring AF (again, because it's so long) but it gets the job done.

The pattern I abused was to use quick attacks at his face, let him cover up, give another punch against his block (not intentional ... just was button mashing), and then pause EVER SO SLIGHTLY. As soon as your opponent looks like he's going to move, jump back, let him whiff, then jump forward again and continue the quick attacks. Rinse, repeat. The slight pause is critical because if you don't, he won't take a swipe at you and chances are you'll get the enraged attack. And, after you get the hang of it, you won't have to watch for him to make his move, you can just give a slight pause that feels right and then move.

Whenever I'd free-fight, I'd end up annoyed that the controls weren't responsive, the hit boxes too generous, or I'd wonder why I didn't dodge where I thought I would. Anyhow, find this pattern and stick with it for three to four minutes and you'll have the championship.

Hold the guard button down during the loading screen that leads into the fight. Get close, then back up. That way your opponent charges you. Block/parry him. Punch 3x (2x Skellige fights), back up. Repeat.
Make sure to always save before-hand and always bet Max! Practice makes perfect.