-Do as many side ops as possible early on because you'll be able to rapidly unlock developments as well as build up your GMP.
-Don't shoot Quiet, literally the best buddy in the game.
-The Sneak Suit is the best outfit, get that.
-Fulton Extract as many tanks as possible.
-The best sniper is the Brennan, it requires a blueprint so do the side ops.
-For the skull missions, use D-Horse with the exception of Metallic Archea in which case use Quiet with the Sinful Butterfly rifle.
-Finishing missions quickly makes getting S rank easier so do that.
-Smoke grenades are very good for dealing with armoured units, grouped together enemies and Total Stealth missions as you can walk towards them whilst they're stunned, CQC them and kill/choke them.
-It always helps to place C4 somewhere obscure so if you're about to get caught, you can detonate it to distract them.
-If enemies find your vehicle, they will destroy it so keep it out of the way.
-If you've encountered a hostile vehicle like a tank or a chopper and you don't have a suitable weapon, use the outpost's turrets.