I play as a sniper and try to use my men in a way to cover all avenues. When I play SP games I always go for the optional objective first, see the route below which reflects how I would sweep down to where the code book is. C is the sniper/demo team and at A I can scope the watchtower and take out the snipers at both the tower and the shack below, where your team can take up its hide.

Scope the Watchtower

By moving your teams thru the passages in the cliffs you can make short work of meeting the objectives. After dispatching the snipers from the watchtower you can advance the team until they are in position to cover the rescue of the Marines (and give you more firepower for retrieving the codebook) and to the V-22 as shown below. This route also shows where the teams can be set up for covering and holding back until the SAM can be taken out, as shown in the second image.

Scope the SAM

Once the SAM is taken out it's just a matter of sweeping south to dispatch the rest of the Cubans. If completed in a similar way to how I have outlined there is no need to enter the extraction zone as you will eliminate all resistance.

Playing as a sniper allows for taking long shots from hidden places where the enemy knows not where you are.