How can I destroy only part of an actor?

I’d like to create a random size block and it can be destroyed by a bullet shot by a character.
I want to destroy only part of the actor, not the entire actor.
For example, like making a hole in a wall or digging ground mobile games.
What I want to do is make a hole on a static mesh with a free drawing shape.

I found Masking is pretty much similar to what I imagine.
Before I found out masking, I was thinking to make a big block by attach a lot of little block and destroy only hitted by bullet.

Please let me know if there is any good way to do this.
And also about how to generate free shape and size blocks!

Thank you in advance!