He was certainly a powerful individual, but not nearly as powerful as he is made out to be. The fact is, Cloud too was injected with Jenova cells, and he wasn't capable of the immense amounts of destruction that Sephiroth was capable of. It was solely because of the fact that Jenova exerted influence over him that he had the power to take his Bizzarro and Safer forms, and solely because of her interferece that he was capable of his most destructive acts. He is still an extremely worthy combatant on his own (as is evidenced by his badassery in Kingdom Hearts) but he's not nearly as formidable without Jenova's assistance. This fight is between Dracula and Sephiroth, not Dracula vs. Sephiroth with some help from Jenova.

The fact is, Cloud was pretty badly wounded too after those fights, and even in that light, he was capable of taking a full attack from Sephiroth while incurring almost no damage while a single counter-strike from Cloud was capable of defeating the villain.

And maybe my memory simply sucks, but I don't recall Sephiroth single-handedly destroying Midgar. I recall Meteor destroying Midgar, and we've already established that Meteor would be a logical impossibility in this fight.