An ugly situation at best...

You could try tearing up the flooring and running floor new floor joists by taking 2x6s and nailing them against the existing joists. The top edge of the new joists would be above the old ones and give you a new level surface to nail your flooring to.

Run a dry line (string) the length of the room, down the center of the room and use that as the level that will give you a flat floor.

The string will sag in the center (just because it does) so you might want to set a leg there to hold it up. Here's where a laser level would really come in handy. You'd get the same level across the whole room with no margin of error. Just find the high point in the room's floor and make that the new elevation for the floor in the room.

Good Luck.

Good Luck.