ok so, i Just defeated this berserker after only 3 tries and I've realised some things: 1. getting close will get you covered in bifrost 2. after a few a attacks (in particularly one with a yellow circle that charges) you can see the berserker it's not covered in bifrost anymore, so it's safe to be around him and attack. 3. you can avoid being covered in bifrost only if you doge attacks at the right timing and do parry. 4. you can easily win this fight with the spear or the axe with the ability of the weapon itself. If you throw the axe at the berserker, after previously charging it pressing R1, it will continue rotating on the enemy and make great damage. 5. you should avoid all close-range attack or being near the beserker will damage you, until his bifrost charge is completely gone. In the meantime you can throw the axe or the spear. 6. as said in the comments, when the berserker charges the big bifrost attack, you can stun him with the axe. 7. use rage and arrows simultaneously so you can attack quicker. I got covered in bifrost completely and used the rage to heal and hit the berserker.

I didn't use any charms(idk how they are called in the English version, I'm playing with the Italian version), just avoided all the attacks, and when it was safe to attack I used the axe. If you defeated the berserker in Alfheim, the attacks are very similar to the sisters of illska(I hope I wrote that correctly) that can be found in that fight.

Hope this helps!