Mirelurks are very tough if you can't hit them in the face
I beg to differ
Whilst their torso is heavily armoured, even absorbing a point blank shotgun sneak critical on occasion, I find aiming at their legs puts them down quite efficiently, this technique is very effective with the lack of a face shot from behind. I don't know the actual statistics, but I'm going to safely assume that other parts of their anatomy are more vulnerable than the torso.
I don't like the idea of using a dart gun, never have, don't particularly ever want to. Deathclaws are terrifyingly powerful, and that's how I like them