Tsyr said:How do I reason with a guy like this?
Well, we could try using reason backed by evidence.
Get out a DMG for him. Open it to page 100 and 101, the section marked "Challenge Ratings and Encounters". Also have him read the bottom of page 101, "What's Challenging?"
Then got ot page 145, "Character Wealth"
That will provide most of your ammunition for the following points -
1) The CR system assumes that the characters follow the wealth guidelines. If the party is not so equipped, they must be considered to have fewer "resources", and can thus not handle encounters a fully equipped party might handle easily. The CR system assumes that the 12th level fighter is likely to have +3 weapons and armor, the 7th level folks should have about +2 weapons and armor.
2) If much of the spell power of the party has already been used on another encounter (the wards), they should be understood to have even fewer resources.
Then ask him if he bothered to think about how the party was supposed to damage a monster with high DR if you had few spells and no potent weapons. I think he will answer that he didn't think about it, that he took the CR on faith.
Then ask him if taking it on faith now seems like a good idea. I think this example will prove to him that CR is only intended as a guideline.