This can be attained on the second act of Oil Ocean Zone. But before starting, whilst in the Mania Mode screen press Y to bring up the Game Options and enable Debug Mode.
To access the Level Select Screen, highlight the 'NO SAVE' option (with Sonic selected as your character) and hold X followed by holding Start.
When the Zone loads up, transform into a Ring by pressing X and follow the route shown in the video to the third submarine in the level (the small one). Press A to scroll through the items available until you reach the Starpost. Place it on the level by pressing Y and scroll through again with A until you find the Large Ring and place a few down. Turn back into Sonic by pressing X and collect the Rings and enter the Special Stage via the Starpost. Complete the Special Stage (by hitting a Red Sphere) and return to the level and enter the submarine to unlock the achievement.