Flexible energy resources are increasingly becoming common in smart grids. These resources are typically managed and controlled by aggregators that coordinate many resources to provide flexibility services. However, these aggregators and flexible energy resources are vulnerable, which could allow attackers to remotely control flexible energy resources to launch large-scale attacks on the grid. This paper investigates and evaluates the potential attack strategies that can be used to manipulate flexible energy resources to challenge the effectiveness of traditional grid stability measures and disrupt the first-swing stability of the power grid. Our work shows that although a large amount of power is required, the current flexibility capacities could potentially be sufficient to disrupt the grid on a national level.