(Apologies for the somewhat vague subject, it's late and I'm runnin' low on coffee

)
I have a question about the scaling nature of the NPC system. Let's say (by way of example) that the party's Big Bad Guy is going to be some variation of a Fallen Angel of the Avenging type. An Avenging Angel has an XP value of 177 which according to the descriptions on p244 is an "Extreme Threat" - also described as "The NPC(s) are party killers".
The party runs into their major adversary fairly early on, and wisely flee the scene. Several adventures (and levels) later, they run into the Angel again.... only it's XP is
still 177, and it could still be described as a party killer. (The problem is even more pronounced for something like a Dragon, the least of whom has an XP value of 184, and goes up to 243(!!!!))
How is a party supposed to overcome these threats? I guess I'm missing something but I don't know what it could be...
Viperion