How can I make a wooden floor flat, without using self leveling product?
Self leveling underlayment is the way to go and maybe there is a way to accomplish that, I have used caulk to create a "dam" to keep it from going places I did not want it to go or touch.

But to answer your question, use bundles of what is known as "undercoursing shingles" they are in essence wood shims that are 16" long, 3/8" at the thick end and about a 1/16" at the thin end and vary in widths in the bundle. If you floor has a consistent drop of 3/8" on each side of a high spot, lay the shingles close together, and so the thin ends are close to the high spot, and the thick ends are at the walls. Hold them in place with staples on the thin ends, if you can get a narrow gauge stapler and compressor would be handy for this. Laminate flooring will not span over any distance, that is why to set them close together. If the thin ends of the shims are 4" apart, the thick ends will be very near or at the walls. The thin ends, with the highest part of the floor between them, should more or less create a straight line between the ends of the shims and the high spot. A little sanding to flatten out here and there would most likely be needed too. I have used this technique under hardwood flooring, a different animal than what you are working with, but with care, you may be able to pull it off.