GUARD 1: One of us speaks only the truth!
GUARD 2: The other only lies!
Mage: Ok I know this puzzle. All we have to do-
Barbarian: *swings his warhammer, rolls a 20, and flattens Guard 2 into a fine paste*
Cleric: WHAT THE HELL, MAN?!
Barbarian: *points at the dead guard* Is that guy dead?
GUARD 1: *sighs* No.
Barbarian: He's the liar!