I'm not sure what the difficulty levels mean, and I didn't see a difference between "mild" and "rock". In both cases, I won easily. Mainly I just took longer in "mild" because I couldn't tell at first how the balloons work or how to actually enter the space station.
One other problem, the balloons appear to spawn in one big batch at first and then dwindle from there. This means that if the player doesn't figure out the mechanics quickly, they'll have nothing to do.