If you complete the campaign on Expert, you will unlock the Casual and Medium difficulty trophies as well (if you haven't already).
Expert mode is not as bad as it sounds. You will not have enemies show up on your mini map. You can not highlight enemies with your binoculars. You also will not take much damage until you need to heal. (You will die fast and not realize where the bullets came from).
As far as sniping is concerned, you will not have that awesome little red dot on your scope like you did on Casual. That means you will miss quite a few shots.
Checkpoints are abundant so you will persevere with time and practice on each mission. Good luck!