Try to hit an entity riding any ride with any projectile shot by some methods like these:
 
[Will hit]: Shoot an arrow with a bow
[Won't hit]: Summon an arrow with a command and let it fall on the riding entity
[Won't hit]: Spawn an arrow with ScriptAPI and shoot it with shoot() method
[Won't hit]: Spawn an arrow with ScriptAPI and hit the riding entity by applying an impulse