Just started playing this and keep wondering if im playing it "wrong".
Is it possible to stealth every encounter/mission or is viable to play the game guns a blazing?
I've been trying to stealth everything bit once spotted it seems like the enemy onslaught never ends.
You can't stealth every mission no, as many story missions have scripted bits to them that you can't control.
Stealth is the best approach most of the time though. Just keep some things in mind:
* If you miss a shot, enemies will hear those shots land if they're anywhere near by. Even if you drop the target you're aiming at but some shots miss and hit a wall behind them you may have trouble. Same goes for shooting out lights or missing shots on targets like alarms. So, pro tip: Don't miss ;p And even when you hit something, make sure that its not within eye or ear shot of enemies.
* You can't move bodies so you'll want to take targets down in areas where their bodies aren't likely to be stumbled upon, at least for a minute or two, as bodies do disappear pretty quickly, especially when you're not looking in that area.
* Your suppressed weapons aren't silent. This is a big one. Generally speaking, silent pistol shots can be heard within 20 meters, SMGs at 25 meters, ARs at 35 meters and Sniper Rifles at 50 meters. (This is why I usually run with a suppressed SMG when infiltrating a base) If enemies hear your shots they'll become curious, though not fully alerted. If there are no bodies for them to find quickly you're probably OK after a few moments. If you fire suppressed shots within range of sleeping enemies they will get up, but short of anything else to raise their suspicions they won't go far before heading back to bed.
* Speaking of sleeping enemies - don't shoot them, for the reasons explained above. Use melee (IIRC its R3 on PS4) to take them out silently in one hit.
* Take out a base's generator, and be on the lookout for more than one generator or a generator complex in larger or multi-part bases. Taking out the generator takes down any jammers, any AA, all lights and the alarm towers. It raises suspicion but only for what seems like a few seconds before the enemies all shrug and go about their business (even in total darkness). You can take out generators at a distance with your weapony, including long distance snipes if you have a good line of sight - and yes, even through the fencing. Note: If you're having trouble finding the generator, switch to your thermal vision - even during daylight hours generators, AA and jammers light up like crazy.
* If you do raise an alert, relocate! Shift your position, do it quickly, put as much distance as you can between you and your previous position, but try and do it without exposing any line of sight to enemies. Once they're alerted they can see like eagles wearing thermals. The only tried and true method to avoid them is to keep obstacles between you and them. They'll most often move to the position YOU were in at the moment the alert was triggered, and sometimes they'll check out the area the alert originated from first before moving to your position using psychic powers. But if you get used to relocating you can fire and move and whittle the enemy down, even with them alerted.
* In terms of your allied AI squadmates keep in mind that its not possible for them to be spotted by the enemy when they're in their default follow mode. BUT, once you give them a command to move to a specific position or to rally on you they CAN be spotted while moving.