The advice above is generally good. I have two things to add:
1) If your players are comfortable with rules, consider giving them mythic tiers. A judicious application of the mythic rules can make a PC considerably tougher.
It may also help with action economy. The mythic rules offer a variety of ways to get extra actions. Non-casting characters may benefit particularly, because many mythic abilities can be done as a swift action -- something not usually available to martials and tricksy types.
The mythic ruleset does have the downside of complicating the game, though.
2) Consider recruitment. That is, perhaps the bad guys are more interested in recruiting the heroes than killing them, which may lead them to pull some punches. Lucrecia in book 3 would be a prime point for this, considering it was written into the adventure in the first place. However, at that point you'd be A) running an evil campaign where the PCs assist Karzoug in rising to power (or supplant him), and B) you'd be well and truly off the rails.