Just an update on how the game went. I decided to pit my party against 19 level 3 fighter orcs (weapon focus club, toughness, +2 bab). I made one of them a level 3 sorceror instead. I then had the two ogres as listed in the book, and the half-fiend ogre leader boss with a couple of fighter levels. And two gnoll archers for flavor. One of the orcs had an extra 2 fighter levels (the sergeant).
So the party had the human rogue scout the fortress in the dark... so when they got close they knew very little about it. They alerted everyone inside to their presence by accident, then attacked. The spirit shaman used wood shape to open up a breach in the wall which everyone poured into. The battle would have been an absolute loss if the shaman hadn't casted "spike growth) on half of the camp. This actually ended up putting down two orcs and immobilizing others. It lasts for hours, and is still going, and covers a massive area. The barbarian couldn't roll a hit to save his life, but the fighter I had to NPC (due to her absence) was killing orcs left and right. The poor rogue got knocked down to zero hp, drank a potion, then got knocked down to 0 again. She might not make it through the battle conscious. The lead ogre hasn't been injured yet, but he's letting his troops soften then up first and has mostly stayed out of it (other than hurling a few spears from afar). There are also another few orcs who haven't attacked for fear of moving (spike growth). They may come into play later. Both of the ogres are down however.
So, given the right circumstances, a party of level 6's can take out 20 CR 3's, 1 CR 5, 2 CR 2's and a CR 7. The cool thing is, they have almost no magical items.. The whole of the party has 1 +2 longsword, a +1 lizardbane broadsword, a ring of tongues, and a few cure light wounds potions.