It's easier to beat on co-op. Nice to have back up, I guess.
Like, does playing it co-op mean it spawns more enemies to make up for it?
Co-op does spawn more enemies. Not sure if they're supposed to be more aggressive though, seems the same for me on solo or co-op.
Or is the difficulty only in getting more points to meet the co-op requirements?
If you're talking about the ranking system like S rank, A rank, etc, then you have to have a higher score for Co-op than you would need for the same rank on solo. I suggest you look at the FAQs guide for the actual numbers.