Actually, I don't think the situation stretches credibility too much. Depending on the circumstances, those dozen crossbowmen could be a threat even to high level PCs.
Consider a case where the guards are "regulars" of about 2nd level, with perhaps a +1 dex bonus and either weapon focus or masterwork bolts, for a total of +4 to hit.
Now our PC is a 15th level wizard who was calmly walking along, minding his own business. Without spells and magic items active, his AC is likely no more than about 12, and his flat-footed AC will be 10. His HP are probably no more than 70 or so, possibly less.
Again, without spells and magic items, those dozen regular guards will hit an average of about 9 times, with the potential to do 9-72 points of damage, or 9-90 if they are heavy x-bows. There's probably one critical hit in there, so make the damage 10-80 or 10-100. There's certainly the potential, even if just a slim chance, to kill the PC. Make the guards seasoned veterans (say 4th-5th level), and it becomes very likely they will seriously injure, or perhaps kill, the PC. This all makes sense to me.
The difference, of course, comes with spells and magic items. These are the arcane wards that deflect bolts, cause attacks to glance off of rock hard skin, blur the target, give the bearer unnatural health, etc. In that context, there's nothing unrealistic at all about the 15th level PC escaping relatively unscathed. After all, how can a regular joe be expected to deal with such powerful magic?