General information on the mission
The twelfth mission of Metal Gear Solid V is relatively demanding. At first, you need to avoid being spotted by regular soldiers (the ones in the power plant and in the HQ), and by the enemy chopper hovering over the mission area. After you make your way into the hangar, where doctor Emmerich is, you need to get him out. This is made more difficult by the necessity to avoid the Metal Gear, i.e. the giant and perfectly armed giant mech. Just like usually, apart from completing primary objectives, it is also a good idea to remember about secondary objectives. You complete all of them in the enemy HQ.
PRIMARY (1/2): Made contact with Dr. Emmerich - The scientist is being kept inside one of the hangars in Afghanistan Central Base Camp. It is being guarded by a big group of soldiers (e.g. ones with walkers), therefore you need to plan carefully on how to reach Emmerich safely.
PRIMARY (2/2): Extracted Dr. Emmerich - This objective is much more difficult than it might seem, at first. Apart from leaving the enemy HQ with Emmerich safely, you also need to watch out for the giant Metal Gear Sahelanthropus, which appears around during your first extraction attempt.
SECONDARY (1/4): Extracted 3 Walker Gears from Afghanistan Central Base Camp) - the walkers are mainly patrolling the western part of the base. You can put to sleep, or kill, the operators and fulton all of the Walker Gears. To complete this objective, you need the CARGO 2 upgrade (fultoning of heavy equipment).
SECONDARY (2/4): Extracted Dr. Emmerich without him taking damage - You can complete this objective while playing the mission for the first time, but you need to be extremely careful, both while escorting him out of the base and towards the end of the mission, which is when Sahelanthropus appears.
SECONDARY (3/4): Secured the Glamour Model Vertical poster at Afghanistan Central Base Camp - The poster is in the building to the South-East of the hangar where Dr. Emmerich is being kept.
SECONDARY (4/4): Secured the blueprint at Afghanistan Central Base Camp) -
Blueprints are inside one of the hangars in the central part of the base and you can walk into the hangar only if you have not raised the alarm (the flashing blue light above the door, instead of the red one).
Since you need to reach the starting point on your own, it is impossible to define the starting time, but try to make it night. Thanks to thin, you will gain a lot, especially while exploring the HQ, because many of the enemies will be able to spot you during the day, at a long distance.
As for the buddy that you should take with you, your choice does not play that big a role - taking the horse will allow you to travel around he map faster, and the dog will help you find guards in the power plant and in the HQ. You do not need any powerful weapons either (e.g. the rocket launcher) because it would be best, if you could entirely avoid detection and avoid destroying the enemy equipment. This also holds for the ending of the mission, when avoiding Sahelanthropus is the only plan that makes sense.