Last week I got my pure barb to level 20. Its a twf build, drow with the following feats:

TWF
Stunning Blow
ITWF
IC pierce
GTWF
Power Attack
Cleave

With 500 HPs hes a bit weak but I use Two bodyfeeder rapiers to offset damage taken and Its the best combo I've found.

I've been able to solo a good deal of high level content using silver flame potions and the bodyfeeder rapiers.

It appears you get 3 additional multipliers on 19-20 hits. you get one at Frenzy II and one at Frenzy III. You also get an additional one while in a death frenzy.

For the most part ive found that you're better off just using death frenzy and rage and not using the regular frenzy unless you're fighting a mob that is immune to crits.

I've been doing between 250-300 points of weapon damage on 19-20 hits with the two +5 bodyfeeder Rapiers.

The additional damage done with a shroud weapon has seemed nominal compared to the control and balance I've had with the bodyfeeders too.

With Death frenzy off you still get the extra x2 crit and with 9 DR you mitigate up to 24 points of incoming damage (with the additional 15 from the feeders).

After killing a few of the red named rares while soloing in the subterrain I realized that Frenzied Berzerker is definitely no joke. I was a little hesitant about giving up improved crit but I certainly feel it was worth it now.