Before you go to the monastery, you must clear the fortress ruins by killing numerous enemies. The best way to get there is to go and slalom next to the country house (M5,9) and then on the forested hill in the center of the map. Look out for the sniper that stations there!
Get to the northern part of the hill and get near the ruins. A convenient way to eliminate everyone is the entrance from the southern wall (M5,10) - lay down the mines in the passages and make some noise.
Now, get on the wall (in the left passage you can find the ladder) and eliminate guards that are there. The rest that must be eliminated can be shot from the above, you can also use the environment. Don't worry about alarms, the units near the monastery won't hear anything.
When it is all over, jump down near the second cannon (M5,11) and blow it up by using an explosive charge. You will save your traps and TNT.