General Tips:
1) Flank: If you see an opportunity to avoid enemies by going around them and moving onto your objective, absolutely do it. There's no brownie points or participation awards going out to Nazi killers today. Many maps you will find opportunities to flank. The first mission of Under No Flag is a big open map, The second mission for Nordlys is an even bigger open map. Most missions in the Tirailleur are very linear but offer huge opportunities to take a side path around the enemy and shoot them from the sides.
2) Stealth: You channel whatever Ninja Kung Fu master you need to but as it traditionally is for most shooters on harder difficulties, don't bother starting a firefight if there's a tonne of enemies, take your time! There's no monster closets that I know of so once they're dead, they ded.
3) Supply Stations: Probably the most important one depending on the mission, keep an eye out for the supply stations or weapon drops throughout the map. In the Nordlys mission there's a Bren chilling out near one of the objectives you go to, much better than holding the fort in close quarters with a bolt action rifle eh?
4) Weapons: Your main weapon should be a bolt sniper rifle, preferably a silent sniper rifle that can be found only in supply stations. With aim assist enabled, that weapon would be your BFF for any kind of engagement, including silently taking out snipers and guards. Your 2nd arm should be a silent pistol, you'll encounter them basically in every section in a random supply station.
War Story #1: Under No Flag:
Section #1: So this bit is quite short and mostly left up to your discretion as to how you want to handle it, there aren't many enemies meaning you can more or less head straight to the hangar as you as you have control of your character, though I will mention to make sure to pick up the internally silenced rifle you find early on. Grab some easy headshots and you'll be golden. If you're wondering where the hangar is it'll be approximately to your 11 o'clock if you're looking from where you start, pretty hard to miss. As soon as you make it to the hangar all enemies will stop chasing you because you'll be moving onto the next section, make it to the AA to shoot down the planes and you're done (I ran towards the right down the runway and a path opened up in the fence for me to grab the AA on the hill.
Section #2: Quite a straight forward mission, three spots to go to. I went to the airfield first, village bombarded via the airplane taken from the airfield and bunker via parachuting and killing all guards via a silent sniper rifle located in the outpost above the bunker. You can take out the first objective at the airfield with the silenced rifle and the second objective where the bombs are with a well placed grenade so clearly whoever is in charge of munitions storage needs to be shot. The radar dishes at the village can be taken out by a single shot each from a panzerfaust (they just don't make 'em like they used to) you can find I believe either at the start or on your way to the airfield. Last but not least clean up some troops around the bunker and sprint in, you will still have aggro once you come out of the cutscene but they shouldn't be hard to kill, grab the jeep and take it back to the start to complete the mission.
Section #3: Probably the most annoying mission in the game. I HATE defend missions. This one is a bit hard to give tips for as they more or less just throw a tonne of weapons at you and tell you to hold out until help arrives. The final wave consists of two panzers and two trucks. I had to use the Pak .80 emplacement to kill the panzers but you could be more patient and do it another way. First take out the trucks with a cannon, then you can play hide-n-seek with the tanks while taking them.
War Story #2: Nordlys
Section #1: Bit of a long section this time but nothing too crazy, you have to follow an extremely linear path to make it to a factory where you'll meet up with mumma dearest and do some cheeky resistance type stuff. How you get there depends on your patience, you can either kill at the camps along your way (there are only handfuls of guards) or you can kill a couple and skate most of the way (which I tend to do from sheer laziness). Once at the factory it's all simple stuff, kill I believe two-three packs of guards and then do some more blowy up stuff. Once that's done do yourself a favour and just skate back to the starting bridge, I died once and it was because my dumbass went sailing off a cliff. Cutscenes will ensue and you'll be tasked with getting to a drop off point while freezing your ass off. Long story short, use your throwing knives and warm up at the smaller camps and you'll be fine, I think I stopped at two camps along the way, don't try the bigger ones, you'll just alert every man and his dog and get shot in the ass.
Section #2: Very similar to the second section from the first war story (must've been running low on budget). I'll keep it simple, at the abandoned mill thingo at the start you'll find a PIAT, basically an OG RPG with an arc so steep that it'd make McDonalds jealous. Fire a rocket at the truck and get on your zoomies and head down to the docks, make sure to grab some Stickies when you see them, you'll need some. When at the docks, head along the road to the right and you'll spot an opening in the fence, you can take the path around the docks, putting some knives into Nazi's Blazkowicz style, throw a wee sticky on ground in the truck room. Jump onto the ice and climb up the ladder near the front of Boaty McBoatFace and throw another sticky down into the objective, hop down onto the ice and before you zoom away detonate them. Head to the vilage, if you head straight from the boat you'll arrive just left of the police station, knife the two guards, do some arsoning in the police station and zoom away to the final bunker. Kill the 6 odd dudes out the front of the bunker (take your time) and you'll get a cutscene. After the cutscene feel free to die once you catch up to the trucks, it'll spawn you close and on your Harley Davidson. Blow up the three trucks and kill the handful of dudes at the depot and you're done!
War Story #3: Tirailleur
Section #1: Most of this war story is a haze to me sadly due to doing it at 3am and it all looking the same. In essence if there's one thing I can tell you about this War Story it's Get a sniper and FLANK. Most of the time flanking will pull you out in a position that lets you overlook your enemies and forces your teammates to move up. You can do this with the big hill you need to take and hold, you can do it with any bunkers you need to capture and so on. The map is extremely wide (like half of the mothers of people on Xbox.) The conquest style objectives are frustrating as all buggery but just hug a wall and pop enemies where you can and eventually your teammates will form a singular brain cell and help.
Section #2: Mostly the same deal. Flank, shoot, repeat. Stealth where you can. I'm not sure if where and when you pop the flare for reinforcements matters but I got spotted immediately so I popped it. Take out the objectives in town and begin your ascent up the hill. On the right side of the hill as you approach there is a rise you can go up and drop down. It serves okay as a sniping point from which you can drop down right next to some ammo and the objective. Same as before, wait for your teammates to figure out the equation and move on. In the next part I just legged it up the hill, you may want to kill folks if you're getting murdered because it's easy to get stuck up there. There are a couple spots for Panzerfausts throughout the mission so you can use them on the Artillery tanks, you won't need them later really. At the top of the hill you'll be faced with a handful of elite troopers. Just take your time, they're slow so use explosives or w/e else you might want to send them to hell. Once you breach the door and do some slow-mo shooting you're all done.