It should also be noted the Paladin can opt for a Cold Iron +1 longsword in addition to bless weapon. If anything they should trade and then the Paladin bless the fighter's sword.
The Ranger's tanglefoot bag option will half of the time ground the Scarlet Son with a little better then a 50% chance of success.
Everyone except the Wizard have +1 weapons so they ignore the Scarlet Sons DR. The Fighter, Rogue and Ranger (as previously mentioned) can have the Cleric or Paladin cast Bless Weapon to deal with the Succubi.
I'm confused by the amount of difficulty your group had...