Marlene didn't have the strength to retaliate once the Cruciatus Curse finally let up. She barely had the strength to curl herself up into some pain-ridden fetal position in hopes of alleviating some of the pain that lingered even though the curse had let up. She wasn't giving up; she couldn't give up, because that would very likely mean that she would die -- these people had killed Moody's mother; she was sure they wouldn't spare a thought for her either way.
Barty's boil spell spilled out across her face before she'd had a chance to put up a half-hearted shield spell. It hurt, and they burned, but at this point Marlene would take looking like some horrible swamp creature over the fact that the spell Barty had cast could have been something much worse.
She pulled herself up onto her elbows, only thinking that she needed to get away until she either had the strength to fight back or to apparate out, glaring the Death Eater and raising her wand only long enough to cast a weak tripping jinx at him before her arm flopped back down against the ground with a thwap.