GoldenEye 007

As noted in the roadmap and a few other places, difficulty achievements DO NOT stack. You must complete every mission on all three difficulties for all three achievements. Because of this, I recommend starting on Agent to get your bearings and see how to do the mission, then Secret Agent, which usually adds another objective or two, and then finally 00 Agent. This will make each mission much more manageable, since you'll learn enemy placement and level layout better as you progress, getting you ready for the next difficulty level. This achievement solution will cover 00 Agent only, since that encompassing objectives from Agent as well, obviously.

On 00 Agent difficulty, enemies do more damage, are more accurate, have more health, and drop less ammo for you. You also typically have more objectives you need to complete to succeed in the mission. The other downside is that body armor that is present on Agent and Secret Agent often is not available to pick up on 00 Agent. This can make things quite difficult in missions with many enemies, as there is no way to refill your health, so you need to avoid damage as much as possible.

Below, I have listed levels with some tips from me as well as a video guide showing the mission being completed on 00 Agent. I have not listed missions whose speedrun cheats are for 00 Agent, as there's no point in doing a casual 00 Agent run on those missions; you might as well go right for the speedrun. Refer to the roadmap to quickly jump to the videos for those missions.

Dam
This is actually a tough first mission on 00 Agent, especially if you haven't played the game at all, or recently. The aboveground areas aren't bad, but the underground area has tons of enemies, and they can come from both sides, and you can't regain health. Take it slowly in the tunnels, peeking slowly around corners while zoomed in with the rifle, so you can shoot enemies before they can see and shoot you. Always check behind you after you kill the next group of enemies, just to make sure you don't start getting shot from behind. You can even backtrack a bit each time just to check, since time isn't an issue here.

Runway
This is markedly more difficult than on Agent because you need to drive the tank all the way to the end of the runway to get the mounted guns, ensuring you don't get gunned down and that the plane doesn't get destroyed. You really need to get use to how to shoot from the tank while leading your shots so they actually hit the guns, because it can be really awkward to get used to. I recommend ignoring all enemies on foot. Drive down the center of the runway (so you don't hit the plane on the right) and immediately take out the first gun on the left. Stop and shoot the missile launcher on the right, near the plane. The two guns in the distance shouldnt be shooting at you. Now head down to the end of the runway and take out both guns as quick as you can. As soon as you destroy the second one, hop out of the tank and run to the plane. Don't bother trying to turn the tank around. Hopefully you don't take too much damage from enemies so you survive the run to the plane

Surface 1
Thankfully this one is a piece of cake. Enemies are few and far between, and seem to have a really tough time hitting you if you stay moving outdoors. You will likely only encounter a handful of enemies in this entire mission, so it should pose no problems

Silo
This is a tough one, because there are a ton of enemies, all in close quarters, and there's no body armor on 00 Agent. There are two options: take your time, slowly peeking around corners, trying to kill each enemy without getting hit, or rush around the corners to catch enemies off guard and quickly gun them down. I used a combination of both. The only time I sat back and was careful are in the longer hallways where you can open a door and back up or go to the side and be careful. When enemies are literally around a corner, I would just rush around the corner and quickly shoot them

Frigate
Most of the difficulty here is the upper rooms, with the maze-like doors and enemies all over, plus making sure hostages survive. With the right route, this mission becomes much easier, because enemies won't be alerted to you, and you'll continually be coming up behind them, making it easier to kill them without getting shot and to save the hostages

Bunker 2
This mission can be really difficult, because enemies infinitely respawn from inside the barracks area. On top of that, if you set off an alarm, you're in for a world of trouble. The best way to do this mission is as shown in the video below: be stealthy. Enemies are only drawn to you if you fire an unsilenced weapon multiple times, not by enemy gunfire, so use that to your advantage, ensuring all the enemies in the level don't swarm you, as that assuredly will mean death. The video below does an excellent job showcasing this very effective strategy

Statue
Another very easy mission, as there aren't many enemies to deal with. Shoot the four in the beginning, then you shouldn't even encounter any more on the way to Valentin. Again no enemies that need to be shot from there to the statue. At the statue, I recommend pulling out your gun, shooting one of the guards near the middle (either the left or right one) as you run straight toward 006, ignoring the other three guards. You may get shot once or twice, but there aren't any more enemies in the mission to worry about. Just put your gun away when you get to the helicopter and you're home free

Streets
This mission can be quite challenging because it's so open, many enemies, and lots of firepower. I don't recommend using the tank, as it makes you a slow moving sitting duck. Make sure to grab the body armor after meeting with Valetin and the second body armor after the left fork. Even with two body armors, you need to be very, very careful here, because a single rocket can kill you even if you have full health and full armor. This mission will definitely take a little luck. If you get to the left fork mentioned above and still have some armor, I recommend just beelining right through it instead of trying to kill enemies. Rush right to the second armor and then right to the end of the mission, only shooting enemies directly in front of you. The video below shows an alternate method using the tank if you'd like to try that.

Depot
Thankfully a very easy mission after the last one. Take it slow, since there are a finite number of enemies, and you shouldn't have to deal with many enemies outside of the two garages you need to clear. This is a short and simple mission

Jungle
This mission is way more intimidating than it needs to be. Once you know where the turrets are, and general enemy placement, this mission isn't too bad. Use your pistol as efficiently as possible until you're completely out of ammo, then switch to the AR33 for Xenia, who you should have plenty of ammo for. Trigger her at the end of the bridge, then back up and aim just to the right of the tree in front of you. Aim at her head/chest and just hold down the trigger, emptying a whole clip into her, and you should kill her before she even reaches you. With her dead, carefully kill the enemies just inside the cave, take out the turret, kill the two enemies at the ladder, then slowly climb up the ladder. Peek and destroy the turet there, destroy the ammo dump, destroy the second turret, and then it's time to rush to the end. Enemies infinitely respawn behind the crates at the end here, so don't bother hanging back. As soon as you destroy that second turret (which is the last of the mission) just rush right through to the exit door. Hold down the trigger to fire straight ahead at any enemies you're running towards, but don't stop moving

Control
Another mission that can be pretty intimidating but it's too bad. It relies on a little luck while protecting Natalya, but the rest is actually pretty easy. Clear the first room slowly and carefully, and there's no reason you should take any damage here. Maybe one shot from a turret, but that's it. Proceed through the next two rooms, letting enemies come to you so you take no damage. Ignore Boris, plant the four remote mines in the main area, go up to the second floor and quickly kill the two enemies there, then up to the third floor for the body armor tucked away. Head down to the bottom and to the side room with the two turrets. Stay back when you open the door and you should be able to see and shoot both turrets without them even shooting at you because you're too far away. Enemies will come when you shoot, so let them come to you and kill them. While defending Natalya, stand with your back to the giant monitor and look directly across at the stairs leading down from the third floor, as this is where all enemies will spawn from. I literally just stood there holding  and slowly firing bullets even if I didn't see enemies. That way, as soon as they come to the stairs, you'll hit their feet and stop them, and then can fire faster to finish them off. If one gets by, quickly aim at them and kill them. Turn up your volume in case you hear the glass break on the right, because that means an enemy is there and is going to shoot Natalya and kill her. Otherwise, the enemies up top will generally only try to shoot you. As soon as the objective completes, ignore Natalya and get up to the second level. Kill all remaining enemies now that they're done respawning. Most don't even care about you. Now you can either take you time with the last two rooms, or after destroying the last mainframe, just rush right to the exit door, depending on how much health you have left

Cradle
This mission can be tough, since you're basically running right at 006 while he shoots at you. Grab the body armor at the start or you stand no chance. The best thing you can do in this mission is keep up with 006 as much as possible, so you can shoot him right as he stops, before he has a chance to start shooting you. He has laser accuracy, so anything you can do to avoid letting him shoot is critical. I recommend using your pistol for all enemies at the start of the mission, until you run out of pistol ammo. You'll want the automatic weapons for 006 while you chase him, and to finish him off at the end. At the end, drop down and go around the platform ladder counteclockwise, as I found this usually results in being behind him. The one time I went the other way, he shot and killed me right at the end