General information on the mission
The thirty ninth mission in Metal Gear Solid V is a replayable one. Its premises are identical with those of the fifth main mission and you can find all the details on targets and the area of operation there. The key difference in this mission is that you cannot be spotted by the enemies, or the mission will end in a failure. This does not go, however, for your buddies. What makes this mission even more difficult is that you cannot use the reflex Mode nor the Chicken Hat.
If you have been playing mission five several times, you should remember the route to the place where the engineer is being kept and you can easily rescue him during the day also. As for secondary objectives, it is best to complete them at night, because you risk more there and you encounter more enemies. It is a good idea to take the D-Dog along, who will reveal positions of the enemies guarding the engineer and will make it easier to eliminate or avoid them.
Armoring does not play a major role here, because you will either have to avoid the enemies or kill them silently. Still, make sure that you wear your best stealth suit (e.g. Sneaking Suit).
Hints on how to complete the mission
The key requirements of Over the Fence remain unchanged. Below, there are a number of hints on the most important modifications, which for obvious reasons, result in the increased difficulty level.
To get to the engineer in Wakh Sind Barracks, it is best to climb onto the rocks to the South-West first, thanks to which you can avoid unnecessary confrontations. Around the engineer's staying place, there are 2-3 enemies, but after D-Dog sniffs them out, you will have no problems avoiding them. At the same time, it should be reminded that inconvenient enemies can be eliminated with the dog and, even if the enemies spot him, you will not fail the mission.
Completing side objectives may be a bit more tricky. First of all, create a checkpoint after you free the engineer, thanks to which you will not have to, practically, repeat the mission from the very beginning. Sneak even if your enemies are standing well away and avoid attacking them, if there are other guards around.