You're right, I was trying to achieve that juice moment where the ball hits multiple zombies and comeback to you! But yeah, there is a lack of polishment and balance. I tried sometimes with less and slower zombies but it became so easy hahaha.
Also, there is a way to respawn the ball (right click), you can do that every 3 seconds, that's useful as well (and it shows that I should create a tutorial next time)