Intelligent attackers can suitably tamper sensor/actuator data at various Smart grid surfaces causing intentional power oscillations, which if left undetected, can lead to voltage disruptions. We develop a novel combination of formal methods and machine learning tools that learns power system dynamics with the objective of generating unsafe yet stealthy false data based attack sequences. We enable the grid with anomaly detectors in a generalized manner so that it is difficult for an attacker to remain undetected. Our methodology, when applied on an IEEE 14 bus power grid model, uncovers stealthy attack vectors even in presence of such detectors.