I used PCGen to make up a challenging heroic "mirror" character to challenge the party fighter. He's a 6th level Fighter with a low-magic shield, some potions, and some complementary feats, and an AC of 27. What's he worth to the party in terms of XP? What's the general rule on this, I was unable to find anything in the core rules...?
CR=PC class levels-1
6 level fighter=CR 5
It is in the gamemastering section
Adding NPCs: Creatures whose Hit Dice are solely a factor of their class levels and not a feature of their race, such as all of the PC races detailed in Races, are factored into combats a little differently than normal monsters or monsters with class levels. A creature that possesses class levels, but does not have any racial Hit Dice, is factored in as a creature with a CR equal to its class levels –1. A creature that only possesses non-player class levels (such as a warrior or adept) is factored in as a creature with a CR equal to its class levels –2. If this reduction would reduce a creature's CR to below 1, its CR drops one step on the following progression for each step below 1 this reduction would make: 1/2, 1/3, 1/4, 1/6, 1/8.
Thanks Wraithstrike. I definitely missed that.
Remember if you give the NPCs a significant advantage, or disadvantage, vs. your players to adjust the CR. i.e. a group of low-level archers with cover and obstacles between them and the players is much higher than their combined CR would suggest!