Stagnox is the boss of the Forest Temple. He is a large, armored beetle. Phase 1: Stagnox will charge at Link. Use the Boomerang to hit the bomb flowers scattered around the arena, then quickly draw a path for the Boomerang to carry a lit bomb flower to Stagnox's back. This will stun him and expose his glowing weak point. Attack the weak point with your sword. Phase 2: Stagnox will shed his armor and fly around, occasionally dropping smaller enemies. Use the Boomerang to stun him mid-flight, then attack his exposed body with your sword. Repeat until defeated. Be mindful of his charges and the smaller enemies he spawns.
