Can the Peerless Swordmistress use her ability when attacking or defending, or just attacking?

"Action: When this unit is opposed in combat, deal X damage to target opposing unit. X is the number of opposing units in combat."

She can use her ability while both attacking and defending. The requirement for her ability is that she be opposed. She is opposed by the attacker when she defends and she can be opposed by defenders when she attacks. I have a great swordmaster deck that uses her.