I indeed let myself believe that if the game was so hard, it's because I played it alone. I was actually very difficult to explore. I had to find every seed after delivering every crop of the previous kind. I lost once or twic, looking for them ^^'
I had to fight so much that I didn't even have time to water all of my crops in a whole day haha! I think it would be great leaving more time or spawn less enemies when playing singlet-player.