This paper aims to find strategic locations for additional Phasor Measurement Units (PMUs) installation while considering resiliency of existing PMU measurement system. A virtual attack agent is modeled based on an optimization framework. The virtual attack agent targets to minimize observability of power system by coordinated attack on a subset of critical PMUs. A planner agent is then introduced which analyzes the attack pattern of virtual attack agent. The goal of the planner agent is to mitigate the vulnerability posed by the virtual attack agent by placing additional PMUs at strategic locations. The ensuing problem is formulated as an optimization problem. The proposed framework is applied on 14, 30, 57 and 118 bus test systems, including a large 2383 node western polish test system to demonstrate the feasibility of proposed approach for large systems.