From Sonic Retro
XX is the final zone in Sonic Advance 2. The level is essentially a "Boss Attack" level, because you'll have to fight all the bosses from the previous zones in quick succession (even Egg Utopia's Egg Frog boss, which you had just defeated). Each one of them takes 4 hits to be defeated (the cannon from Hot Crater's Egg Bomber Tank and the laser from Sky Canyon's Egg Saucer need only 2 hits to be destroyed). After getting rid of the Egg Utopia mini-boss, the exhausting run across the stage suddenly stops and you face the final boss of the zone, Super Eggrobo Z. If you finish this zone with all the Chaos Emeralds using Sonic, you advance to the True Area 53, the real final level of the game. If you don't, then the normal ending plays and the game finishes right there.