In terms of his skin being nearly invulnerable - the butt-bomb is not an attack where an object is trying to pierce or break the skin. It's a matter or tremendous force and power being universally exerted outwards. To the degree that there is a "weakest link," one doesn't have to find and attack it like this. The pressure and shockwave from a massive explosion will naturally exert, find and focus on any areas of weakness.
Think of an actual grenade. You would probably have a hard time, with a knife or screwdriver, finding a way to pierce the outside of it. But it's designed for an explosive charge, from the inside, to burst it.
I'm also reminded of Robert De Niro's solution to penetrating the safe and the metal chambers he couldn't cut in the movie "the Score" - he created a tiny hole to the metal shell, filled it with water, and then set off an explosive charge from the inside which created too much pressure for the strong, impervious, metallic outer shell to contain (which is, essentially, the exact scenario we're talking about with Translucent).
Here's a scene from Myth Busters where they test the scenario from "The Score" -
YouTube: Mythbusters - Safe Cracking (at 4:07)