Hold your Defenses — never burn them at end of turn just to clear your hand. The Lord and Loyalists need them most, since they draw the focus fire first. The lower your HP or the more enemies eyeing you, the harder you should hoard. But it only answers Attacks and Hail of Arrows: a Duel or Barbarian Horde still demands an Attack, not a Defense.
Zhao Yun loves this card: Dragon Heart swaps his Attacks into Defenses, so nearly every card in his hand can save him. Eight Trigrams armor rolls its own Defense on a red judgment, letting you bank your real Defense cards for when the armor whiffs. Shadow Runner stretches the distance so fewer Attacks reach you, meaning fewer Defenses burned.
To break a wall of Defenses, flood them until the hand runs dry — Zhang Fei and the Repeating Crossbow attack without limit, forcing Defense after Defense. Lu Bu is nastier still: his Peerless fury makes each of his Attacks cost two Defenses. Or sidestep the card entirely with things it cannot answer — a Duel, a Barbarian Horde, or a Fire Attack. Thin their hand with Steal or Dismantle before you strike.