2. Vary Your Approach - Guards Will Learn Your Tactics
When you finally get hands on the Combat Unit and the deployment options therein, this will become less of a problem, but one of the most devastating things the game doesn't tell you is that the enemy outposts will actively learn from your techniques in the field. If you repeatedly strike at night, they'll don night-vision goggles, if you're a headshot champion, it'll be reinforced helmets, an action-heavy approach and they'll have better assault rifles, and so on. There's not a whole lot you can do about this other than varying your approach so you don't develop a trend the game can latch onto (it will to some degree for the sake of this feature regardless), and if you find yourself going up against a fortress where everyone is a specced-up super-soldier spotting you from a mile away, it might be cause to put the Combat Unit to good effect and stop them. Phantom Pain does mention you can send your troops out on missions, but doesn't actually ensure you do so. To get the best out of this function, dive across into the 'Missions' tab of the iDroid, into 'Combat Deployment', and scroll down past the suggested missions for your skill level. Look to the icons that represent things like rifles, helmets etc. and providing you've got around five B+ soldiers in your ranks, deploying them on these missions will ensure the enemy doesn't have a leg to stand on next time you're around.