People says that the Berserker is so hard and they can't defeat it easily.
I just beat it with characters: Ryu 43 lvl, Rei 41 lvl and Peco 42 lvl (Third character is pretty useless.) And this is how I did it:
I bought first like 50 Soul Gems from Faerie village (used about 10).
Then I equipped Ring of Fire to all my team-members. (Stolen from Red Drake in Myria station.)
I found Berserker in Container Yard near Caer Xhan as usually.
So my units and items were:
Ryu: Dragon Blade, Dragon Shield (I stole Aries Gloves from Berserker during the fight and equipped them), Dragon Helm, Dragon Armor, Soul Gem and Ring of Fire.
Rei: Slicer, Mind Shield, Sun Mask, Royal Armor, Soul Gem and Ring of Fire.
Peco: Brass Claws, Spiked Gauntlets, UV-Glasses, Amber Breastplate, Sould Gem and Ring of Fire.
When the fight begun, immedeatly I transformed into weretiger with Rei and influenced Berserker with Ryu.
Berserker used Fire Wave (or something like that) with no use on me. Only scary thing was its attack which did like 350-700 damage to my members so it killed my every member with one shot. Berserker uses also berserk-move but it doesn't bother me because it kills me with one shot without berserk.
So fight is very simple: put back lost Soul Gems and attack with every powerful attack. I used Rei's weretiger, Ryu's supercombo with focus and Peco's Myollnir. It died in 15 min or so on.