They are quite powerful if you lack heavy damage reduction, hitpoints, they have the magician's head, and they can prevent you from fleeing. Lightly prepped characters are easy lunch, because the Battle cabal is specifically meant to offer entertainment for the 2k hp air/offense ABSS scions.

Berserkers typically favor damroll/strength builds, leaving them more vulnerable to mental attacks (Did you notice how there were four bards in Empire around the time of the Rites?) and supplications if they neglect spell saves. Scouts and defenders take a different approach, but your complaint is against deathblow.

Cloud giants, arials, and what's-his-name's-gnomes are the only villagers who fly, leaving many vulnerable to quicksand, pits, and trip. Their inability to use magical transportation, healers, and healing preps means that sapping their movement is a valid tactic in many circumstances. It should not be surprising that antipaladins, necromancers, most rangers, evil conjurers, and shaman all can attack movement. Transmuters and shapeshifters instead have superior movement capability.

To fight berserkers one on one, consider the following tactics:
1. attack when they don't have the Head.
2. catch them when they are crippled
3. melee: haste, aura, shield, stoneskin, reduce/enlarge. Mages: add barrier. Communers: add protective shield.
4. If they like to rage, lead them to or summon powerful mobs for them to rage into. Nothing like trying to defend near the giant and encounter Orumous the Strong.
5. And of course if you aren't a prep fiend, don't attack them solo, as that is stupid.