Hi, I was hoping someone else would answer your question.
Sometimes big questions or big requests are hard to answer. It is important to learn the fundamentals of playmaker.
Did you have a chance to learn how to use the raycast action from the video? Use the raycast to "shoot". Find out what gameobject it hits. Then send an event to that gameobject (enemy) with a custom global event. On this enemy, use the the action "knock back". The raycast will give you the position of the hit normal. Use this position for the knock back action.
Action:
https://hutonggames.com/playmakerforum/index.php?topic=14171.0Or you can apply force if you want to use rigidbody physics.
Here is alternative way to do knockback as well. This one instead of using a normal, calculates the direction between two game objects.
https://hutonggames.com/playmakerforum/index.php?topic=16400.0Let me know if this helps.