Can someone please help me with this issue? https://www.youtube.com/watch?v=-rMWYgjU1a4
Hey @BlennzFire! Welcome to the forums!
That looks like artifacting due to them being box brushes, still. Once you have the thing that you want built, you want to grab them all and right click - > convert to static mesh, making them all one piece. 
That should get rid of the artifacting! You can also have much better collision and be able to block light much better than a brush.
Hope that helps!
I have already done that once and it didn’t fix the issue sadly.