- You may be looking for a different Challenge Act, 200% Boost Gauge Challenge.
300% Boost Gauge Challenge is the fifth and final Challenge Act of Green Hill Act 2 in the console/PC version of Sonic Generations. It is unlocked after clearing every Stage in the classic era and is playable only as Modern Sonic.
This Challenge Act takes place within a slightly modified version of Green Hill Act 2. The level's platform and enemy placement have been slightly changed, and the player begins just after the cave section where they would be chased by the Giant Chopper. Outside of those changes, the level is mostly the same as the regular Act, although the Goal Ring can instead be found at the beginning of the stage's final 3D stretch.
In this Challenge Act, Modern Sonic must run as fast as he can through the Act; this time he starts with his Boost Gauge being tripled. However, the Boost Gauge cannot be replenished by any means. The Challenge Act starts with large sections of 2D platforming, in which the Homing Attack can be quite useful, but Sonic enters a 3D section near the end. Regardless of the camera angle, the player must use the Boost to their advantage with precise timing to reach the Goal as quickly as possible. Once this Challenge Act is completed, the "Boost Gauge Up" Skill will become available in the Skill Shop.