They are the Septuple compressed cobble from XU, so no, he cannot break it. And this isn't my build. The obsidian should be fine. I generally kill my first wither using a similar build in the nether using the bedrock ceiling, so I make a warded room using TC's warding wand focus.But wouldn't the explosion destroy blocks in AS once he does his thing? Would obsidian work in that case to war off the exlposions? As for your build I see what it does, or rather how it does it, but having trouble piecing together how to build it.
Sidenote: @Padfoote You are really handing out the likes.