Clever ways to reduce an encounter CR

Question

My current dnd group has been playing for years, while I am relatively new. Unfortunately, it seems that none of them have really ever put much stock in arcane/divine classes or abilities (this includes the current DM).

Our DM has recently decided he wants more arcane in his campaign and has asked me to help him organize magic heavy encounters (I am the closest thing to an arcane expert as we have, as far as rules and strategy go). I have given him lots of ideas for NPC arcane battle synergy, but it seems like my encounters are are always overshooting his target CR.

What are some general ideas for reducing difficulty by 2-3 CR without rebuilding the monsters. Lets say we take 2 Water Nagas (7HD) straight from MM. That would make ECL roughly 9. How can we get it down to 6 or 7, without changing their stat blocks directly?

I was hoping for something on the lines of this: when you approach the encounter, the nagas are already engaging some crabs. Then they will be softened up a bit, and down a couple of spells.

Or basically give the PC party one free surprise round.

Do these get closer to a CR of 7? How far away am I still ? any other imaginative ideas?

Answer 1

The way I do it is pretty simple: Give party opportunity to prepare. If monsters are busy, party knows about them in advance and so on, effective challenge rating drops. When scout reports monsters vulnerable to fire, and wizard has time to prepare fire spells, or at least find fire scrolls to have them at hand, and fighter can switch his +2 greatsword to usually less effective +1 fire longsword before fight, first rounds gets way easier.

I mean giving them an extra opportunity to do so, and making extra tools available, as seamlessly and unobtrusively to the storytelling as possible. Standard scouting and preparations, without active help from DM, should not lower effective CR.

Other option is quite similar to the one you described in your question: Monsters, especially evil ones with Int worth speaking about can quarrel with each other. Can fight with each other. They can refuse to cooperate with each other if party is smart about approaching them. Think Drows: "letting him die may be good for my position in House" type of thinking - Drows may choose to fight with less than their maximum effectiveness. This strategy is pretty good in three ways:

• it makes encounter easier
• it adds some depth to the villains and world
• it teaches players to expect surprises (he wasn't as incompetent as he wanted to look like when you killed his older brother, after all)

Answer 2

I would say that if you cannot find fitting low-enough level monsters you could always try turning low-level monsters into Wizards/Sorcerers/whatever using the rules found in Savage Species (p. 8).

As far as I know there is no way to "unadvance" monsters in the same way you advance them: you could always try reducing their HD, though this is not a 1=1 scale with CR. I would say find how much DR each HD on a monster is worth and tinker with that.