The second and third cannon
Go along the coast (M6,6) and head west to the starting point. On your way, you will only find two guards. Next, use the stairs to get to the ruins on the hill. Watch out, one of the soldiers is an radio operator and the second carries a Panzerfaust - you should eliminate him, soon, his weapon can be very useful. In the remains of a tower farther from the stairs you can find additional equipment (ammunition, traps) (M6,3).
Look out for the sniper in the distance (M6,3) - eliminate him when a cannon is shooting or do it later. Go around the ruins and jump down into the bushes (M6,7) - before that, from the red chest, near the place from where you jump down, you can take 20 suppressed rounds for your rifle (M6,4).
The officer is in front of the bunker (M6,8) - he is one of the main objectives. Shoot quietly the orange aggregate next to him and get rid of the officer. After that, enter the bunker and blow up the cannon by using charges (M6,9).
You can now walk through the cliffs and surprise the enemies from the other side or simply get inside the second bunker unnoticed and detonate the next cannon (M6,10). On your way north you can jump on the lower rocks and walk along the coast - no one will see you.
The fourth cannon
Behind the first bunker (M6,11) there is a minefield (M6,1), a good place to get additional traps. If you are afraid of disarming them, you can go along the fence - the mines are only in the center of the road.
Go higher. There are two enemies in front of the bunker and another two on the stairs. A stone won't distract them but destroying a large bottle/chest on the right will do the job. Set the charge near the cannon (M6,13) and go back to the previous bunker behind the minefield.