Marcy uses Cat'sCradle on Serge.
Cat'sCradle is a tech used in Chrono Cross learned by Marcy after 3 boss stars are acquired. Since 3 stars are received before fighting her in a boss battle, she knows the tech during that fight and before she enters the party. When cast, she throws strings of sticky web at a target. The target is snared by the adhesive and suffers damage.
Cat's cradle is a child's game played by lacing string between the fingers to form patterned nets. Often the game is accompanied by nursery rhymes. Since Marcy is decisively young and child-like in both manner and appearance, it is no wonder her first technique reflects this persona.