Sorry for this rant post, but those berserkers are so fucking annoying! I am playing on GIVE ME A STORY and still they just never die! The fucking health takes thousand strikes to lose a fucking millimeter!
Leave them for later.
Optional top tier bosses like Berserks and Gna are designed to face them in the end game, once you completed the game and explored the map so you leveled up enough to highly increase your health and rage bar, found better armor and weapon items and leveled enough to increase your strength, defense etc. stats enough, unlocked more useful special attacks and level them enough, etc.
Also when facing them make sure you choose your proper build that gives you the more appropiate stats, attacks, etc for that particular fight. Then think about the best fitting and more effective attack patterns and when each one is better and in which order. Also, obviously learn to block and evade properly.
I platinumed the game and got the 100% after 92 hours of playing the game. I killed only one before completing the main story, and left most of them until I completed pretty much not only the story but also most secondary/optional stuff. By doing that and facing them in the appropiate moment and with the appropiate tools, it's a 1 or 2 minutes fight each.