This is the kicker right here. One reason parties can theoretically fight monsters at lot higher level than they are is because most monsters don't have any blanket (you can't touch me) abilities. No unbeatable SR, no save or die if your X HD or lower, no DR 20/epic, etc.
But some monsters do, and those monsters have to be treated differently.
For example, someone posted that a 23 level monster could be taken down by a few 100 1st level characters. But if that monster had had even weapon resist 5 or fast healing 1 it would have been a completely different ballgame.