Take this with a grain of salt because I do not know everything about the Frenzied Berserker:
Is the following what may happen?:
1. FB attacks enemies wildly, in a Rage, with many extra hit points.
2. FB takes enough damage to be at -11, but is still on his feat because of his Raging extra hit points from temporarily raised Con.
3. FB kill all enemies and is still Raging - he missed his Will save. Or, alternately, he makes his will save and dies because he stops Raging and his Con drops to normal.
4. FB is "Calmed" via Calm Emotions or other means. FB dies because he stops Raging.
Or:
Party attempts to heal FB. First, a touch attack is needed because the FB is avoiding the heal, thinking he is being attacked (he's still Raging, after all). FB takes his AoO with greater Power Attack and kills the cleric who is trying to heal him.
FB stops raging and dies as he is below -10 hit points.
I'm think that, properly role-played, a FB is not survivable because of the above.
The FB might only be viable as an NPC. That certainly seems to be true if it is role-played as in a bezerk frenzy when Raging.