So, how would this work? It sounds like two powers that are absolute. I would say that Cosmic Spider-Man would win out, but I'm not certain.

WS064 Cosmic Spider-Man
UNI-VISION: Cosmic Spider-Man's line of fire is never blocked.

vs.

EW049 Green Arrow
THE DARK KNIGHT RETURNS: NOT HIDING ANYMORE: When Green Arrow occupies clear terrain, he can use Sidestep and opposing characters can't draw lines of fire to him unless they are within three squares of him or he has 2 action tokens.

Green Arrow. He's not "blocking" CSM's LoF , just not allowing him to draw it.

NeoShazam has the right of it.

Awesome! Thanks!

Now...Cosmic Spidey vs. RE031 Batman sounds like Spidey wins, yes?

YEAH. HE DOES THAT.: Lines of fire drawn to Batman are blocked unless the line of fire crosses only unoccupied clear squares.

Yes, you are correct.