The approaches below are using QGIS 3.2.3-Bonn. Screenshots and other features may change or not be available in other versions.
Traditionally contours are labelled with the labels always upright in the uphill direction. Depending on how your contours have been generated, you may need to use the
Reverse line direction tool in the Processing Toolbox to regenerate the contours in the other direction.
Under Layer → Properties…, go to the Label tab
Label with = ELEV (assuming that is the elevation attribute of your contour layer)
Under Buffer sub-tab, select
Draw Text Buffer, which will create a thin highlight around each contour.
Under Placement sub-tab, select
Under Rendering sub-tab, select
Show upside down labels = always, and
Suppress labeling of features smaller than = 30mm (or as desired). The
Show upside down labels is important if you want contour labelling to be consistently in one direction.