No need to go into complex mathematical operations, just use the encounter rules. An elite monster is worth 2 PC, so a level 23 monster count as 46 levels of encounter, so it needs 46 level 1 PC to make a good challenge :p
In the glossary p.46
It seems that damage resistance will be against a specific kind of attack instead of being against all attack except a few exceptions.
Also, on page 47, there is Vulnerable X damage, which add extra damage from a kind of attack. The troll example above posted by mhensley...