Levelling a bathroom

Started on my sister's bathroom yesterday. There are floorboards which were cupped slightly, but I did my best to flatten them with belt sander and plane. One corner though is a little low as a result of restumping. I would say about 4 to 7 mm. How does one level (or go close to level) the bathroom without taking off all the floor boards and starting over again. Would you worry about the height that it's out or use more adhesive (in the low corner to raise the height) as she will be putting tiles over the fibre cement when I put it down?

put the tile underlay down, then water proof then use leveling mud (or is it the other way around?) then glue down the tiles....

Pretty much what he said.