Alright, so this isn't a new chapter; just wanted to clarify something.
No, Doomguy is not nerfed in the story. He's still powerful as usual. My assumption, however, was that his "invincibility" also included "cell regeneration". And the reason why he may seem like he's having difficulty fighting against Deathstroke is that he's acting charitably. Mind you, he still threw Batman's ass around and kicked him to the curb earlier.
Also, him bleeding becomes a major plot point later in the story.
In future chapters, however, he will be invulnerable to whatever the villains hit him.
So yeah, there's my two cents for you.