never saw him lose in the comics the mask i mean but he was hurt by a voodoo doll but not like hurt hurt, it just caused him pain, some stuff poision ivy put in his face made him do what she wanted, some magic arrows messed with him but when he got them out that was it, and a gilt bomb from lobo made him get messed up the thing is with things like poision ivy and the gilt bomb they affected the wearer not the mask himself, its complicated, when someone threw him into a portal he just came back dont know if it was under his own power but yup he did come back, the level of power you get using the mask depends on how crazy you are, joker made the mask give him a nuke, lobo killed whole galaxies with it, etc etc. but for sure no one can kill that thing never seen anyone hurt it and when they do manage to effect it even a little it just heals super fast afterwords.