Ghost Rex is the first boss of Sonic Rush Adventure, appearing at the end of Plant Kingdom, the game's first real level. Presumably a creation of Eggman Nega, this robotic dinosaur is encountered stomping through the fungal forest, while Sonic explores on his first Wave Cyclone trip.
Like all subsequent bosses, the Ghost Rex's fight arena is 3D in contrast to the 2D platforming of the game's regular stages. The robot prowls back and forth on the ground, occasionally lunging at Sonic and slamming its head to the floor. Strike it by attacking the weak point for massive damage; in Ghost-series robots, this involves a spin attack to the glowing red control jewel, here embedded in the Ghost Rex's forehead.
Once the health bar is cut down to half, the dinosaur flies into a rage, stomping the ground so hard that the surface gives way, dropping itself and Sonic down a huge pit into an underground cavern. The boss continues underground, although now the robot telescopes out its neck and brings it crashing down faster. Just keep dodging the slams and jumping on its head to prevail.