Did Deathstroke really have no way to deal with Darkseid as he was now?
Of course not. Even with a casual thought, there were several methods available within the DC Universe alone.
For example, he could teleport Little Cousin over, then offer him up to play a "three-person game" with Lucifer and Maze in exchange for Lucifer making one move. This Black Lantern Tyrant would definitely be finished.
Or he could contact Sepulk and have them reach out to Saint Killer. After all, Su Ming had helped that guy get his wife and daughter back, with a Heaven thrown in as a bonus. Getting him to fire another shot should not be difficult.
He could also contact Death through his own Black Lantern Ring and have her repay that favor. That counted as a Finishing Move too.
Or he could use the Nightfall Greatsword to break open the planet's cage, wake Hal up and bring him back, then join forces with Sinestro. Three White Lanterns against one Nekron would be a guaranteed win as well.
There were plenty of tactics, but Su Ming simply did not use them, because this was all Darkseid's conspiracy.
No one could say for certain how many avatars this Dark Tyrant had, nor what the situation beyond the multiverse was like. If he used these plans so early, what would he do next time?
Leaving everyone else aside, that gay Lucifer would never be willing to help every single time.
Darkseid wanted to use the advantage of having more cards in hand to forcibly trade away Deathstroke's cards, leaving Deathstroke with nothing to play in the future. It was a reverse version of the boy who cried wolf.
If all the favors he could call in were used up, what would happen when the real