Shadow is able to fight against metal overlord, and Metal overlord is way stronger than super sonic in sonic heroes, to the point that he needed the team work power to beat metal overlord.
You know what that means?
That the growth of this guys is so strong that even if Solaris was the strongest in the game continuity by 2006, currently, he could be beaten easy by Super Sonic in generations (with the problem of him being omnipresent and inmortal the only real reason for him being imposible to defeat).
And The End is the strongest villain currently, boom, no feat about DESTROYING EVERYTHING.
Pure scaling, the only thing you need.