When used the next attack's damage increases 100% Fierce Brute is a Super and will only be used once. It does not affect thrown weapons. If you have this effect activated and counterattack, it doesn't end, so you CAN use it several times in a single battle: first you activate it, then use something like Deluge which doesn't spend it, and on your opponent's turn you can counterattack him with the full force of this Super. After that, your normal attack will still benefit from it.
The downside is that you have big chances of wasting your fierce for nothing, e.g. when your opponent blocks attack or dodges. Or your brute can just select weak weapon instead of taking something useful.