Day 3 of the Black Hazard event on Kamen Rider City Wars and I decided to try co-op because there's no chance in hell I'm getting 1000 medals just from solo (getting around 35 per day solo).
One of the people that joined my room was using Hazard, so I guess it's safe to assume co-op is a source of potentially unlimited grinding.
Now I just need to not die while hosting a room, because apparently only the host needs to die for it to be game over. That was a waste of 25 stamina. :I
EDIT: Also, any 5-star card seems to count towards allowing you to obtain a rider without spending R-Energy. I just looked at the list and Knight and Hibiki were free despite my never having pulled a 5-star for them from the gacha. But I did have 5-stars from their events, so I guess any 5-star counts.