EGG-HVC-001
From Sonic Retro
EGG-HVC-D01 is a bipedal mech built by Robotnik in Sonic CD. It is the game's first boss, appearing in the Future (Bad or Good) of Palmtree Panic Act 3. Unlike most bosses, he only takes 3 hits.
This mecha has two large arms with bumpers on the end. A hit will take off one of the arms, and after both are off he's completely open to the final attack. It's best to hit him when he's charging up to attack; or, if you don't mind sacrificing your cache of rings, simply throw yourself into the mech's spiked feet, and then wallop Robotnik's pod while you're flickering. Thirdly, you could simply go to the exact right of the stage (before Robotnik appears), so you can easily hit him from behind, beating him in less than 5 seconds.