This guide is going to assume that you are playing on the hardest difficulty available. The strategies presented still apply to the easier difficulties.
The first chapter is by far the hardest, so prepare yourself. It's fairly simple at first, you need to push the objective. Keeping a distance and being patient isn't going to work for this mission, or the next two so get used to slowly progressing as you take out enemies and take cover when needed. The first objective you're given is a good time to start practicing this new strategy for future use.
Make your way towards the hill, and find a bunker somewhere on it. Get inside there, and hold your position here as you take the objective. You'll have to defend it for a second too, do that, and then begin the hardest test you will face in this entire campaign.
There are three cannons firing upon your position. You are tasked with destroying these. Big problem is that these things can fire from a massive distance, and they aren't exactly the next door neighbors. Their also defended by plenty of infantry, with an assortment of weapons. So you will need to run and gun the first part. Once you get close enough you should get a checkpoint, but the real challenge hasn't even started.
At the bottom of the hill you have to push up on, there are tons of infantry slowly retreating. In front of the right and left cannons are two bunkers armed with two mounted turrets each. And down the middle either set of turrets can and will target you. Behind these three cannons is a fortress, with a mounted turret on top, and three more cannons to keep you busy. You have to destroy all three front cannons before you're given another checkpoint.
I spent hours analyzing every single part of this point in the game. Mostly because I had no choice. The enemy spawns weren't always consistent in their timing. Meaning sometimes I would clear out one area, just to have some of the initial enemies spawn right behind me after a delay. This delay was random, and unpredictable, making the cannon hill one of the most difficult fights I've encountered in an FPS.
After testing my luck, pushing my limits, and getting very frustrated, I came up with a solution that finally pulled me out of this abyss of difficulty. What I ended up doing, was pushing the left cannon first. When you get up to a cannon, your allies will start to push. However, I noticed that they tended to always push the left and middle parts, and not the right. Pushing the right first would cause them to run directly into gunfire from the middle and left, and render them completely useless.
So pushing the left they were able to actually get into the area with me, and from there they would flank the middle from the side. You can find dynamite close to the bunkers and either side, and for the middle cannon it's very close by. I would clear out the enemies in the left bunker, grab the dynamite, hold my position in the bunker for about three to five minutes, and then blow up the left cannon.
With my enemies keeping their focus on my allies on the left side, I then would run back down the hill, carefully avoiding cannon fire, and then once at the bottom, I ran to the right side and back up. I repeated the process for the left side the same way on the right side. It worked like a charm, and then I would run along the path behind the right cannon to the middle one, and blow that one up as well. Finally completing this part of the mission.
It's important to note that I discovered the thing that killed me the most, was the mounted turret on the fortress behind me. I would recommend keeping a precision weapon on you so you can take out enemies periodically as they get on this turret, as it can end your run pretty quickly and mercilessly. Also, destroy any explosive barrels you see from a distance. A stray bullet can cause you to get blown up by these if you're too close.
Now take the fortress, and then you'll have to defend it. There is dynamite lying around the fortress, I would recommend planting this on the pathway into the forest as soon as possible. This can allow you to prematurely blow up either the troop transport or the tank that you'll have to kill in a little bit. There is an RPG and ammo for it in two different locations in the fortress, at the top behind the mounted turret, and towards the back on the left wall. Use this to take out whichever vehicle you aren't going to use the dynamite on.
When infantry are overwhelming you, camp out in the bottom section of the fortress. They normally charge one by one, making them easy targets. Finish defending, and the first and hardest chapter, will finally be over.
The second chapter is the easiest in this campaign, so get ready for a breather. This chapter has two parts, one where you're running through the forest either quietly or loudly, and a second part where you're storming another series of objectives.
For the first part, I just stuck to the left and assassinated the first two guys I saw. Then I just started booking it. You'll get to a bunch of debris lying around, just run up the fuselage and the next part will start. I did this while being chased by enemies, and was able to completely avoid fighting them by just running past.
The second part is a bit trickier, but just requires some trial and error. There's some dynamite lying in front of the first AA gun, you can easily toss it over the wall it's in front of and blow up this first objective. From here, just figure out what path best suits you based on your weapons equipped. This is another section where you have to push or you'll just be wasting ammo. For me, what worked was moving up the left path past the first AA gun, then going down the middle, then up the right. You'll have to take another objective, defend it, and then this chapter is over.
The third chapter starts out pretty unforgivingly. You have to infiltrate a village that is guarded by two cannons and a tank. There's also a comms tower which infantry will use to call in reinforcements. Worst part is, dying will reset you to the beginning of a rant that your main character goes on for quite a bit. After hearing it for the 20th time it gets pretty irritating.
What I did was kill the guy that will be walking in front of you silently. Then I ran down careful to avoid being seen. On the right side of the house closest to you is some dynamite. Take it, and then I moved quietly to the other side of the house and planted it on the ground. I went back around, shot the comms, and then I ran inside the house, up into the attic, and shot my flare up out the window and into the sky.
This next part is a bit of luck. The tank needs to move over your dynamite before it gets destroyed by random gunfire. I don't know if this happens rarely, because this took way too many tries for me, but eventually it did happen. I blew up the tank, and then started taking out infantry. The nice thing about having this area open is that the house in front of the first one gives you great cover from the far cannon.
I'm not really sure if this is consistent, but the closer cannon that you're also supposed to destroy was almost always getting blown up by something else. I'm not sure if I just kept getting really lucky or something, but I never once had to go out of my way to blow it up.
For the farther cannon, once you've cleared out the central village area of remaining infantry, head left outside the gate. Near a small shack there's an RPG. Aim carefully and quickly. DO NOT give this cannon a chance to kill you.
For this part, you can run backwards without having to worry about enemies following you. So do that if you're feeling overwhelmed. Your buddies can help you out here too. They actually will kill some of the enemies, and act as great distractions for the far cannon.
Once you've gotten passed this, it's mostly smooth sailing. I would run along the outskirts of the map until you're behind your enemies. This worked very well for me as your allies will push up based on how far you've progressed, and infinitely spawn too. So running behind your enemies causes them to get overwhelmed by your infinitely spawning allies. Get to the gate of the castle and shoot the bad guys in the back with absolutely zero honor. After this, shoot some baddies in slo mo, or don't. The last cutscene will play. It's a long one too, but definitely worth the watch. Absolutely heartbreaking.
If the achievement for beating all the campaigns on whichever difficulty aren't unlocking for you, I'd recommend rewatching the Under No Flag ending. This forced it to pop for me. Hope this helps! And be proud of yourself! This wasn't an easy feat to accomplish!