Projectile kills can come from a number of sources. They include, but aren't limited to at the time of writing this:
Dogs (X to hit it into the air, and then X, O, or Triangle to send it in a direction)
Bloated Bro, the big zombie wearing a yellow jersey (spam Triangle)
Barrel (hit it once to send it flying, then again choose from X, O, or Triangle)
SWAT (Triangle to disarm the shield, then choose a direction)
Shockwave Titan (O, O, X to send the head forward)