Answering your question as is:
Glue a 3/4" inch block to one side of your ply wood, place a 1/2" block at 1/3 of of the way, and a 1/4" block at 2/3 of the way and then run the 36" sheet of wood thru a planer, cutting a 1/32" off each time... Might be able to do it faster, but planers eat plywood for dinner so, slow and steady wins the race.
Purchase self leveling concrete, mix it up as directed then pour it on your floor, make sure you get it to 3/4" on the side that is not level and spread it across the hallway... Let it cure then put flooring over it.
Not the prettiest solution, but it works.
A more expensive solution
instead of using self leveling concrete, use a 2 part, thin plastic resin, it will not grind to dust like the concrete, will bend and flex (depending on the type of resin) with the subfloor, and can be screwed through more easily. It will just cost an arm and a leg. Though this cost can be reduces by putting strips of wood, or some other filler into space to be leveled with resin to reduce the amount of resin required...
Advantages: Can get much thinner then self leveling concrete. Will not crumble.
Disadvantages: Cost... Also, also future remodelers will look at what you did and wonder why in the ***** you did this. Will either take far to long to cure, or not long enough... Slight danger of fire... Will make the hallway unusable for a long period of time. Also might summon Bob the god of Builders to smite you for heresy.