Upon entering a large open area, you'll be ambushed by several waves of Greebles. Some will attack directly, while others will perch on ledges and throw bombs. Prioritize the ranged Greebles first, using your Skylander's ranged attacks or quickly closing the distance. Utilize any environmental cover available. After defeating the initial waves, a larger, armored Greeble will appear. This enemy has more health and a stronger attack. Focus your attacks on it, dodging its charges. Once all Greebles are defeated, the path forward will open, usually by a gate retracting or a bridge extending.
