The Egg X (エッグ X) is the seventh and final boss in Sonic Advance, which takes place in the X-Zone after having defeated the preceding two bosses.
Dr. Eggman is protected by a spinning red ring with three weapons fitted to it: an arm, a ball cannon and a laser beam. This red ring covers the cockpit at all times when he isn't attacking, and will stop and lower when he uses one of four attacks:
- By shooting a ball at the player, with two different firing angles. They must jump to avoid it.
- By firing a laser beam at the player. They must duck or time their jump to avoid it.
- By grabbing the player with a giant hand, then shake their Rings away and hurl them to the other side of the screen. They must jump to avoid it.
- By not using any of the weapons and instead flying towards the player. They must jump to avoid it.
The player can only inflict damage on this boss when Eggman's cockpit is exposed during an attack. It's easiest to damage the boss during his grabbing attack, as his cockpit will stay open the longest during this time. Hitting the cockpit when Eggman charges toward the player is also effective, but attacking when Eggman shoots a ball or fires a laser is not recommended as both attacks come out very quickly, leaving little time to strike the cockpit. Amy, like the Egg Hammer Tank boss, has the easiest time damaging the "Egg X" with her Hammer Whirl attack ( + ), and is capable of getting in AT LEAST three hits at a time before the weakness is concealed again.
The player will have to hit the cockpit eight times (six times if in Easy mode) in order to destroy the machine, causing the entire space station to explode and resulting in the ending. However, should the player defeat this boss as Sonic with all the Chaos Emeralds and after having completed the game with all four characters, the Egg X will stop exploding like a regular boss, and Eggman will retreat to the Moon for the true final showdown with Super Sonic.