Ok, this idea is terrible, but...
It's a kaiju.
You have to fight a giant monster which has an emerald embedded in it. You can block, which uses some of your shield, do a special move, which drains your regenerating special meter, punch, kick, and you only need to win one round to get the emerald.
As the stages progress, you need to fight bigger and bigger monsters, and you can even choose different abilities before you fight. In one fight, you can use a dummy ring bomb, or in another you can do an epic spindash into the monster.
The design of the stages will change when you go to different stages, like in one stage there's a checkerboard pattern ala Sonic 1, or a ruined city like Angel Island's.
tl;dr: It's not good. It's never good. It's just kaiju.