Here is a short video by me to be level 40 fast in Sniper Elite 5.
It is possible to level up to 40 just by playing the whole campaign.
However if you want to boost here is a technique.
Just save in front of a place that can bring experience and reload your save after the action. Experience counts even if you repeat the same action over and over.
Here for example at the very end of level 6 (Douvres). I took several levels in just a few minutes.