Below is some Calculations I made about the end bosses in the sixth book (The Empty Throne). I would like to get some other ST opinions.
Part of my issue (and I wonder if some people have had the same problem) is that the value of gold for the PC's equipment far exceeds what they should have at their current level (average of 199,400 per character at level 13. I have 5 players) I haven't really given out too much extra, and they have even spent a bunch of money on their caravan, gifts to npcs, buying all the ninja clans, and even buying a miracle to save the wife of an NPC! (book 5, the evil socererer's wife - I wonder if anyone else's party has done that?) My party is very efficent, and so it is often a struggle to give them a good challenge.
So to get back to topic - I will have 5 PC's and Ameiko going up against 4 enemies with a value of 960,000 to them. I am thinking of drastically upgrading their gear to equal what the PC's party would be if they were all level 15. I would like to hear thoughts on this, and if anyone else has had a similar problem.
I would rather recommend adding more NPCs to that party, because the action economy already will work against them. I am more or less in the same boat as you are, with six players in my group.
Also, check attack bonuses versus AC values. Since my guys are very defense minded, it happens a lot in my campaigns that the attacks of even high level NPCs/monsters have a very high chance to miss. If that is the case with your group, too, think about upping attack bonuses on the NPC melee characters.