Tips to Obtain Snowball Achievement
The achievement of hitting 2 or more zeds with one Ballistic ball isn’t properly working. It is not written, but you must hit 2 or more zeds without killing them.
So a easy way is just go to any close / indoor map, wield a door and let the zeds break it.
Then, just tap the bounce ball over and over, it will stumble but not kil them outright. You might be able to do it in 1 wave, if lucky.