Duel ignores range and can't be dodged with Defense — only Attack answers it, so it lands guaranteed damage whenever you hold more Attacks than the target. Aim it at light-handed strategists like Zhuge Liang or Zhou Yu who rarely keep Attacks. Rebels love it to bleed a far-off Lord without closing the gap. Just count your own Attacks first so you don't lose your own Duel.
Guan Yu is the dream pilot — his red-to-Attack conversion means he almost never runs dry mid-Duel. Zhao Yun can feed Defense cards in as Attacks to keep swinging. Lu Bu's peerless skill forces the opponent to answer with two Attacks every round, making him the undisputed Duel king. Diao Chan can even bait two rivals into a Duel against each other.
Negate cancels the whole Duel before anyone bleeds — hold one back for when you're the target. Otherwise keep more Attacks in hand than the challenger, or let Guan Yu's red-card conversion carry you so you never run out. Take the 1 damage and heal it back with a Peach or Hua Tuo's mending. Against a Duel-happy Rebel, don't burn your Attacks on your own turn — save them.