How could I flatten these warped sleepers layed on uneven concrete to an acceptable level?
Perhaps you're thinking about this in the wrong order.

What if you applied the plywood to the almost level sleepers and then leveled a surface over the ply? 

I think you''re going to have a very difficult time leveling sleepers as you have in that photo.

EDIT: Per your comment that you intend to install tile atop ditra.... you have a very forgiving tolerance with the subfloor, then!
Glue and screw the ply into the sleepers. Then apply the Ditra.
You can level out imperfections in the leveling with thinset below the tile depending on just how bad things are.