All enemies have 90% Combat Advantage. Normal enemies must stand on opposite sides of a player to deal this extra 90% damage, but bosses deal this additional damage at all times. This means that Awareness is just as effective as Defense at reducing damage from boss fights.
To determine how much extra damage enemies will deal with Combat Advantage, subtract your Awareness Total Percentage from 90%.
For example, say a character has 50% Awareness and a boss is attacking for 100 damage, not including Combat Advantage damage. The character will take: 100 damage * (1 + 90% (enemy Combat Advantage) - 50% (player Awareness)) = 140 damage in all.