Not sure why everyone is looking for a save, not that it's possible. Even so, riot mode is easy. I've completed it with 55 waves with almost 890k points.
I noticed that the "High Scores" leaderboard is saved on the GameSettingsV2 file, so I've just made a save after getting the 50 waves trophy and tested on a second account by completing 1 more wave and then quitting/dying/restarting.
Except for the trophy for completing Riot Mode for the first time, nothing else popped - not even the other cumulative trophies, unfortunately.