Well I will help you just cannot explain right now as I am no where near my PC. Hopefully someone else will jump in before me. If not, I will definetly help you in the next 12 hours. Verbally, it would take me too long to type it all. I hope you undetstand. But basically, it will happen as explained
to change a static mesh you simply drag out static mesh component in your blueprint actor and set static mesh to whatever you want. You can trigger this as a function via an Event hit from projectile like in the video. For the effect you simply spawn emitter at location.
Offcourse, this is a very coarse way to put it.
Edit - It is obvious that you will need two meshes. One without hole and one with hole