Streethfighter/Begulier or Soul Blade seems nice for solo damage spike. Spamming Phatom Foes (to get enemies "Flanked") and Mind Afflicted powers will allow you to get full advatage of crazy damage bonuses that Streetfighter has:
"Bonus: -50% Recovery time +50% Damage against Flanked, Mind Afflicted or Body Afflicted targets, when the street fighter is Flanked or Bloodied. +100% Weapon Crit damage when the Streetfighter is both Flanked and Bloodied"
So you can get crazy -50% Recovery, 50% Damage and 100% Weapon Crit damage.
Phantom Foes is foe-only, so you'd need to be Confused to use it on allies. (For example, Berserker-Cipher... unfortunately Serafen is limited to vanilla Barbarian.)
However, the Blunderbuss "Powder Burns" modal applies "Flanked" to the user.
On Berserker-Storm form: it's not good because the attacks are ranged and the bounces hit your own allies.
Um what? You want your enemies to be flanked unless descrption of Streetfighter is wrong. Read "-50% Recovery time +50% Damage against Flanked, Mind Afflicted or Body Afflicted targets". Against Flanked targets. So you get that 50% dmg bonus AGAINST flanked targets. Hence- Phantom Foes. Why would I want to flank my own allies?
Streetfighter gets bonuses to Sneak Attack and recovery time when Flanked or Bloodied, and bonus crit damage when both Flanked and Blooded.
But Sneak Attack applies to Flanked or Afflicted targets, which might be what's confusing you.
"Sneak Attack becomes more deadly and Recovery decreases when the street fighter is Flanked or Bloodied."