Rocket launchers are conspicuously placed (even on the harder difficulties) right before encounters where they might make things a bit easier. Emphasis onmight.
Since the game as a whole lacks Mook Chivalry on any difficulty higher than Normal, you're completely invincible while doing the Zandatsu animation and for long enough afterwards to leave you plenty of leeway to parry attacks enemies were already winding up.
Most of the boss fights have something that makes them drop health or health packs, the only one that doesn't is Sundowner, and you really shouldn't need healing for him. note It's possible to Zandatsu the helicopters he summons, but they're floating over a bottomless pit, making this arguably more dangerous than doing without.
Stealth
Throughout the entire Metal Gear series, starting from the very first, you can always get unlimited ammo by getting said ammo, leaving that part of the map and go back in for more. You never really run out of ammo unless you decide to just keep going. Likewise in most boss battles, the game will have ammo that will respawn in case you run low on them.
Similarly, the cigarettes and cigar often serve as a poor-man's replacement for items that can be missed or run out, but are reasonably required to beat difficult sections of the game. They allow you to see infrared sensors in case you missed the thermal goggles, and starting with Metal Gear Solid 2: Sons of Liberty they also steady your first-person aim in case you run out of Pentazem. Cigars in Metal Gear Solid 3: Snake Eater help you see in the dark in case you missed the Night Vision Goggles or Torch (or your battery ran out), and also repel hornets in case you ran out of bug repellent. In all cases they never run out and you start with them, at the cost of them not functioning nearly as well as other options and of course they sap your health.
One particularly confusing puzzle required you to "check the back of the CD case" to find Meryl's codec frequency. Not the CD item you found in the game moments prior, but the case the game itself came in. Anticipating that some players might not figure it out or might not have access to the case (often was the case when you rented a game or bought it used), the devs made the frequency 140.15. This way players who gave up and decided to just start at 140.00 and systematically try every frequency would find it rather quickly. Failing that, if you contact Colonel Campbell enough times once you reach the Tank Hangar, it will automatically add Meryl's frequency to your contact list.
During the fight with Liquid Snake atop the ruins of Metal Gear you have a three minute countdown to win the fight, but your opponent continues talking for a solid 30 seconds of it before the fight actually begins. If you die and continue, however, the game gives you some lenience by letting you begin your second attempt with a full three minutes rather than two and a half. Those 30 extra seconds actually make a ton of difference.
During the fight against Psycho Mantis, calling Campbell enough times will have him reveal the solution to preventing Mantis from reading your controller inputs; plug your controller in the second controller port. If for some reason you can't use the second controller port, calling Campbell again will have him provide an alternate solution in beating Mantis, which is to shoot the statues in the room. Naomi chimes in to explain why such a tactic would work; the statues bear Mantis's face which is wrapped in leather, so removing said leather would reveal his ugly mug that he despises seeing, thus breaking his concentration.
In the tanker chapter, if you have 4 out of the 5 photos Otacon needs, but the timer is nearly expired, Otacon tells you to forget about the last photo and get out.
There's an Escort Mission where you have to guide Emma to the Central Core and she's completely defenseless. You can't use Rations on Emma to heal her, but if you let her sit down to rest, she'll slowly recover her health.
On some difficulties, Snake will offer to shoot down the drones that are attacking Emma while she makes her way across the water, as long as you have them in your rifle sights.
There's an anti-frustration feature built into the guards' AI. When an enemy spots you they will run off to take cover and call for backup, at which point Alert Mode is activated and reinforcements arrive. Running into a loading screen (like transitioning between maps) before the enemy finishes his call to HQ resets the map as if nothing happened. Since the game grades your sneakiness on the number of times you enter alert mode, and not the number of times you are caught, some speed-runners use this as a shortcut. However, getting caught by a CYPHER or surveillance camera would instantly activate Alert Mode.
This game gave you the option to swiftly fast-forward through codec conversations by holding triangle, addressing the issue in the previous game where your only option to skip past a codec call you didn't want to hear was to tap X over and over until you finally got to the end. As some of Rose's conversations can drone on for upwards of a half-hour and are about relationship drama that iscompletely irrelevant to the plot, this was a very welcome fix.
During the battle where you hold off waves of Arsenal Tengu with Snake, he will use his M4 and have better AI on low difficulties and use his USP with worse AI on higher difficulties: on Very Easy he basically wins the fight for you, while on Extreme it's basically an Escort Mission where you have to keep him alive. However, if you enter the fight with less than half health, Snake will always use the M4 with better AI no matter the difficulty. Since you can't backtrack to find more health at this point, this prevents the mission from being Unintentionally Unwinnable.
During the segment where you have to protect Emma with a sniper rifle, you can use the Thermal Goggles to spot enemies more easily. The setting sun can be seen in the background and if you use the goggles while looking at it, only the sun itself is illuminated rather than having the player being completely blinded by the sun's light.
Anticipating that the Sniper Duel with The End was going to be very prolonged and difficult, they added a number of ways to make it easier and even outright bypass it. The End will never kill you, instead using non-lethal weaponry and simply carrying you back to a prison cell when you lose, which allows you to stock up and try again without losing progress. Typing the Konami Code at the map screen will show you The End's current location on the map which effectively renders him a Zero-Effort Boss. Though it's tricky, you can snipe him after a cutscene and actually kill him, preventing the fight altogether (you face the Ocelot Unit in the boss arena instead). Most hilariously, you can save during the fight, wait a week or adjust the clock, and resume to find that The End died of old age.
The Updated Re-release added an extra camera mode that gave you free control over the camera, in direct response to fan complaints that the original's sky-view locked camera made it way too frustrating to keep track of where enemy soldiers were (as well as allowed them to more easily see you than you could see them).
Snake's inventory is completely cleaned out when he's captured at Grozny Grad, and even though he gets all of his gear back after he escapes, he'll lose any live animals he'd been keeping. If you lucked out and caught the very hard-to-find Tsuchinoko earlier, another live specimen will be sitting out in the open in the very next jungle area that allows you to scavenge for food.
Metal Gear Solid 4: Guns of the Patriots features a rather difficult Sniper Duel against Crying Wolf and her Mooks, where the developers once again included an easy means of defeating her. Hiding under the truck is a guaranteed safe point where Wolf and her goons will never find you, giving you a safe place to run and hide in if she gets the drop on you. A patient player can even camp under it and, albeit slowly, take her down without any challenge at all.
If you have to restart a mission from a checkpoint, the locations of every enemy you marked beforehand will be retained, allowing you to use past experience to formulate a new strategy.
Reflex Mode was designed as this, granting Snake Bullet Time when noticed by an enemy guard so the player can take out the guard before he triggers Combat Phase. This doesn't work against anything that sees you running around in plain sight, however.note An enemy with a white "!" notices Snake while he's sneaking, and Reflex Mode can be used. An enemy with a red "!" sees Snake while he's in plain sight and plays the proper sound. This can be also be turned off for extra points.note Even if turned on the points are still given if it never activates in the level.
When aiming, Snake will track an enemy in his sights if it's within 30~ meters. This can also be turned off, but there's no bonus for doing so.
Once again, there is a tactic to easily win the otherwise difficult Sniper Duel against Quiet. This time you can take her out effortlessly by calling a supply drop on her location which will lay her right out without you having to even fire a single bullet.
Averted in the Extreme Mode version of the fight, where she can no longer be affected by this tactic at all.
