The Justice League would have trouble with Kuurth & Nul, maybe Mokk with the stone power or maybe Greithoth could get some Kryptonite use against Superman so you would have that & magic against him, the big trouble would be when the Serpent arrives, since it's all prophecy & stuff with no Thor since this would be well in my head in the DC Universe there might not be a way to stop him. Another thing I wonder who of the DC Universe would the hammers have picked given the opportunity.
Hard to believe that it's dead, it's a part of the cosmic hierarchy, a whole part of the working of the universe. If it is somehow dead it could have bad repercussions for the universe, isn't it necessary for the universe, it's like getting rid of something like death or eternity. The universe will be out of balance now.
That's comic writers for you constantly re-writing power levels, histories, one minute a character is mildly powerful next minute he/she/it is insanely powerful. Especially when dealing with high level beings fighting each other, one character with near-omnipotent or omnipotent vs another similar character who wins it very hard to say, depends on the writer. Its tough.
He is the type of character to fake his death to make it easier for him to pull string & manipulate things from behind the scenes, Wolverine, SHIELD & whoever else won't be looking for him if they think he is dead.