The Green Hill Zone Boss is the first boss encountered in Sonic Jump. As is also the case with every later zone's boss in the game, players will encounter this boss in the 6th & 12th Acts of Green Hill Zone, after first completing the prior Acts.
This boss sets the tone for every boss in the game, as all later bosses follow this distinct formula (with the only difference being that future bosses carry various weapons and methods of attack). The player will find the boss lurking in the background in the beginning of the act. After a few seconds of lurking, the boss emerges at the bottom of the screen, pursuing the player with a force field. The goal is to stay above Eggman and jump high and quick to avoid him. After some time, he will boost upward in an attempt to charge at you. Avoid the attack and hit him from below to send him scurrying back to the background. You must repeat this for 2 more hits to defeat him, and the act will loop infinitely until you do so. After defeating the boss, you will find the cage of beloved animals waiting to be freed. Reach the cage to end the Act.
Again, note that all later bosses in this game follow this formula, and require a similar strategy as well. The only difference is that later bosses will have additional weaponry equipped. This first boss carries no such weaponry, and is therefore a good warm-up for future boss battles to come.