One of the classic Flash games and a definite good one!
Since the information doesn't seem to exist on Newgrounds, if you want a shot at getting to Zone 7 and thus a shot at even facing the Corruptor you will need to play in Heroic difficulty only! You will also need to get through the entire game without ever using the training fights, even beyond Captain Hunt and all the way through the Mayor. Only if you do that does Zone 7 open up, which is in no way indicated you would need to do that at any point in the game, so good luck discovering that by "accident".
Once you've unlocked Zone 7 by doing the above, you can then go use as many training fights as you want to grind up and become extremely strong for some of the absolute toughest remaining fights in the entire game!
If you don't do the above, then the game ends when you defeat the Mayor and you don't get to play Zone 7.
How do I know all this when I don't (yet) have the Predator or Over the Ashes medals? It's because I remember playing this game years ago before I ever signed up for a Newgrounds account and this information was mentioned elsewhere. I don't remember if I might have done it with the Hydraulics class though, which would not be an option on Newgrounds. So you'll have to somehow manage with one of the two classes Newgrounds allows.
Hope it helps anyone wondering what was up with that final medal and how on earth when there was no Zone 7 (unless you do the above)