757 damage in one second? Let's see...
Rank 3 (being really generous with that) magic damage per second: 325
Fidd's ult ap ratio: 0.45
Fidd's quoted ap: 100
Actual damage from one second in ult: 370
Okay now for a real analysis: I know this was early/mid game because the fidd had only 100 ap and you were talking about winning/losing lane so he probably had just rank 1 ult.
Fidd's rank 1 ult base damage per second: 125
same ap and ratio
actual damage per second from fidd: 170
So now we know you were in that ult for the entire duration because it would take the full 5 seconds to inflict that much damage and kill you simply based on raw damage numbers and not even calculating defenses