Tempting as it may be to get in, get out, and complete the mission as fast as possible (especially if you want an S rank), MGS5 rewards - literally - players that take their time. Almost all settlements - from lowly guard outposts to high-security Soviet prisons - have an abundance of materials and diamonds to collect.
Get as many of these as you can. The diamonds can be used to buy new weapons, and the materials you find are what goes into making them. Keep a steady stream of these on the way to Mother Base and you won't be caught short when you need a crucial item for a mission.
Back to TopWhile you're in an outpost, you'll notice that there are anti-air (big antenna) and back-up equipment (big satellite dishes) which you can take out. These can be destroyed with C4 and grenades: doing so enables you to call in the chopper for extraction without leaving the outpost, and stops the enemy being able to call in backup. Crucially, you'll need to destroy these (and eliminate/Fulton every sentry) in order to capture the base and make your passage through Afghanistan and Africa that little bit easier.
Back to TopAn easy way to gather intel is to interrogate at least one soldier in every enemy outpost. Once you've got an interpreter (you'll acquire one in the course of the game) you can understand what the guards are saying. Grab one from behind with R2/RT when close and then hold L1/LB and click 'Spit It Out'. Doing so will usually see them point you towards hidden diamonds, specialist soldiers who can help build Mother Base faster, or other items of interest. This is especially useful when you're in a large camp and don't have a positive ID on a target.
Back to TopAfter you've unlocked Mother Base and its various branches, you'll want to make sure that you're focusing on the right tech to see you through the game. There are hundreds of things to be researched, from camos to weapons, but the ones you'll definitely want and need are as follows:
Rocket launcher development
Binocular upgrades
Fulton recovery upgrades.
The rocket launcher is a recommended item (the game even says as much) as early as mission 8, and requires an R&D team level of 12, plus resources and money, to build. With the right approach to tranq/Fulton-ing guards and picking up loot it should be attainable by the time you get there, but keep an eye on the R&D stat.
Similarly, upgrading binoculars is a must. The reason? It takes the guesswork out of finding good soldiers: simply tag them as normal and it'll show you their abilities. Upgrading the binoculars yet more will enable you to find higher-level goons. This can take a lot of work out of the early game. Upgraded binos need R&D and Intel teams to be upgraded (an example: third-stage binos, which can pick out soldiers with up to A++ rankings, requires an R&D level of 16 and and intel level of 14), but keep an eye on your Mother Base rankings to make sure they're always progressing.
Lastly, the latter-stage Fulton is a mission critical mid-late game item and so you'll need to get Support and R&D levels up to 16 and 18 respectively. You know what to do, and with upgraded binos you should be able to get there quickly.
Back to TopThe computer will automatically sort acquired troops into different branches for you, but sometimes - especially if you want to level quicker - it's necessary to sort them yourselves. For example: the game may have put an A-ranked medical team member into looking after injured troops, but they may also have a B or a C in other fields. Switching them out into other fields helps boost timeframes and get you more stuff, quicker. The unit they're switched from may downgrade, but you can always move them back.
Also, the game's main characters - Ocelot, Miller, etc - are great for these, seeing as they're talented across the board. Throw these guys into your most needed area of development and you'll fly up the stages, but keep grabbing troops from the field or you'll leave yourself spread too thin.
Back to TopAmazingly handy as MGS5's slow-mo 'Reflex' system is, there are times where you'll be seen and, in the few seconds you have to react, be carrying exactly the wrong items. However, it's possible to switch between lethal and non-lethal weapons while pressing Square/X with a weapon drawn.
Back to TopIn the early going of MGS5 you'll be tasked mainly with assassination: wiping out high ranking Soviet generals, or eliminating heavily-armoured enemies in the region. Kaz will tell you to kill them, but it's actually better if you don't: kidnapping the three generals with the Fulton in the mission 'Red Brass' gives you better intel on Afghanistan and your presence there, while also gifting three highly-skilled soldiers. Apply this broadly across your targets and you'll soon have squads of elite soldiers back at base.
Back to TopEach soldier you send back to Mother Base has a range of abilities, but some also have specific skills to be exploited. Others, however, have the 'Troublemaker' attribute, meaning that whichever unit they're assigned to will soon see fights breaking out, potentially sending other soldiers to the medical bay and downgrading the unit while they recover.
The key to stopping this is to put two troublemakers into the same unit, cancelling each other out.