I have a fairly clever player whose character recently died. He decided to make a new one, whose thing is being an Eldritch Disciple Halfling that throws a Returning Vorpal Dagger around. As a reminder, here's what Vorpal does:
This potent and feared ability allows the weapon to sever the heads of those it strikes. Upon a roll of natural 20 (followed by a successful roll to confirm the critical hit), the weapon severs the opponent’s head (if it has one) from its body. Some creatures, such as many aberrations and all oozes, have no heads. Others, such as golems and undead creatures other than vampires, are not affected by the loss of their heads. Most other creatures, however, die when their heads are cut off. A vorpal weapon must be a slashing weapon. (If you roll this property randomly for an inappropriate weapon, reroll.)
That in itself isn't a real big deal.
It becomes a big deal when he casts Surge of Fortune, which does this:
At any point before the spell expires, you can channel some of its remaining power into a single instant of perfect fortune as an immediate action. The result of the next attack roll, saving throw, skill check, ability check, or spell penetration check you attempt is treated as a natural 20, as long as it occurs within 1 round of the time you invoked this power. (If you use it for an attack roll, you must still roll to confirm the critical hit normally). Using this option instantly ends the spell.
The party knows the area they're in is occupied by an Ancient Red Dragon, and I don't see how he can't one shot it with that dagger (combined with Power Critical and True Strike to make confirmation easier, which he can do because he has Divine Magician). Daggers are "P or S". Given a throwing dagger/knife is thrown by spinning it, the slashing motion needed by vorpal seems to be satisfied fictionally and mechanically.
I would prefer to not have the "boss" taken out in one hit, but I also am not that keen on house ruling away what appears to be an entirely legitimate build. What I am looking for is suggestions on how an Ancient Red Dragon could defend itself against such an attack. At the moment the Dragon has no way of knowing about the attack, but it is possible it could learn before the party engages (if they fight something else and one of its spies sees it, for example).