This achievement requires completing a minimum of three dread games and a fourth game you'll probably do in nightmare difficulty, so most of this solution is a general dread & nightmare guide, but it also covers the four legacies.
Let's get one question out of the way first - "Can I start out in normal difficulty, then switch difficulties to complete the legacy?" No, probably not in the way you're thinking. When you change difficulties, you are effectively bringing your entire community and its storage locker to a fresh Day 1 version of your map. So, you're back in the starter base, all plague hearts are reset, you have no allies, nothing has been surveyed, etc. It's not a bad way to ease into dread mode for the first time, but don't think you're going to skip huge parts of the game in doing so.
The #1 key thing to remember is that when you choose your community to start the game, the characters from your previous playthroughs will bring not only their skills, but the contents of their backpacks and rucksacks with them. Always keep in mind, when you're about to complete one game, which three survivors are going into the next one and load them up with the things that will help you. I'll have more on that in just a minute, but first, since this achievement requires completing the game four times on dread or nightmare difficulty, I thought a general dread/nightmare guide would be in order. It's long, so you can skip it if you want and go straight to the part of this guide about the four legacies. (Search on *** to jump there.) But if you want to read my general dread/nightmare guide, here it is:
GENERAL DREAD/NIGHTMARE GUIDE
Weapons and Enemies: Your biggest threats in dread and nightmare are, in order: 1) ferals, 2) juggernauts, and 3) hostiles. The reason ferals are the biggest threat is because they are the hardest to avoid. When you see juggernauts and hostiles, you can usually choose whether to engage and prepare yourself first, but you better be ready for ferals anytime, anywhere. In nightmare mode, there are feral packs (three ferals) that can shred two of your most badass survivors apart in seconds. For me, the best way to counter this was the 10/22 carbine. This weapon is relatively lightweight, fires very quickly, uses cheap ammo, has a scope, and is suppressible. The variant obtained in the Aunt Prepper quest has a 50-round magazine. Other variants can be found in scavenging or bought from traders or enclaves, but they are all good. Give this gun to whoever is going out of the base and keep it equipped, and these feral packs become a mere nuisance, rather than a game ender.
Crossbows are also important, and despite all the diferent varieties of crossbows, they all boil down to two kinds: the Echo X-3, and all the others. The Echo X-3 uses lightweight bolts and has a 10-bolt magazine. It is your best friend in clearing out infestations because you can take out multiple screamers quickly, before any of them goes off. It can also save you from multiple ferals in lieu of the 10/22 carbine. It is a rare item that can only be obtained from traders. If you don't have the X-3, then use whichever other crossbow you like, but always carry one.
Juggernauts can be taken down with a few 50-caliber rounds and/or with frag grenades, pipe bombs, etc.
The best way to deal with hostiles is to avoid them. If you have to take them on, bring zombait and bloater gas and let the zombies do most of the work. Use the 10/22 carbine and/or crossbow you should always be carrying to add to their problems.
You'll notice that the only ranged weapons I mentioned were the 10/22 carbine, the .50-caliber rifle, and crossbows. In my nightmare playthrough, I had ample amounts of these and simply didn't use any other weapons.
Resources and Facilities: In dread mode, resources are a little more scarce, but there's still plenty, with one exception: parts. In normal difficulty, I never paid attention to parts, but in dread, I found I needed to hoard them; i.e. always buy them from traders, salvage broken weapons rather than repair them, use a CNC mill, etc. In nightmare mode, parts were always my most serious, and often critical, resource shortage. Parts are needed to produce toolkits, C4, ammo, meds, and much more. That's why one of my biggest recommendations for dread and nightmare is to buy the Red Talon Workshop. To get it, you must have the Daybreak DLC and play it until you have 2,500 prestige points. Then you have to purchase it from a Red Talon trader who you summon with your radio.
The Red Talon Workshop is regular a Level 3 Workshop that can be built in one construction job with a minimal amount of materials. So, it will save you a lot of initial construction time and materials. Additionally, it does passive repair of weapons in your storage locker (melee and ranged) without consuming any parts. I cannot stress enough how important that is in dread and nightmare difficulties. Additionally, it is self-powered, meaning that if you have a CNC mill or other upgrade that requires power, you can use it even if your base doesn't have power.
In nightmare difficulty, the resource shortage is more real. Survivors in your base eat 2 food per day, skipping out on zombie assaults on your base (what the game incorrectly calls "sieges") costs 3 ammo, each gas can costs 2 fuel, and all facilities require many more materials for construction. This means the fertilizer upgrade for your garden/farm/hydroponics is more important than ever, and it makes the rare recycling skill (daily materials & parts income) mighty handy to have. You can make ammo from parts. It all comes back to parts.
Another very helpful facility is the sniper tower, which gives you an awesome radio command (sniper cover for 50 influence), and also reduces the threat level. You can have a sniper tower and watchtower reducing threat at the same time. The sniper tower requires a builder leader. Now that you can demote leaders, it shouldn't be a big deal to choose a builder as the first leader of your community so that you can build the sniper tower. Then, when the plague hearts are destroyed and you're ready to proceed with a legacy, choose the leader you really want.
Skills: Not all skills are equal. You need a computer person to upgrade the command center. Specialize him in electronics to make C4, which is the best way to destroy plague hearts. You need a gardener to boost yields in your garden. A chemist/munitions person can make thermite, which is effective against plague hearts and infestations. A medic can heal you over the radio, which is pretty huge if you've used up your painkillers and can manage to climb a roof or container to catch your breath. A mechanic can make toolkits for repairing vehicles. You'll need lots of those, trust me. A utilities person can build a hydroponic garden. A cook is nice to have, but not necessary. I never needed a craftsman, ever. Avoid recruiting actors, musicians, self-promoters, etc. If you find a rare skills trader, some of the useful rare skills include recycling and soundproofing.
I upgraded all of my cardio skills to Marathon. In dread/nightmare, you're going to be doing a lot more running and a lot less standing your ground. Also, if worse comes to worse (which it will), you can drop all your stuff and sprint all the way across the map without getting fatigued. I also upgraded all my wits skills to Stealth. The main thing in dread and nightmare is to not get detected. Being able to sprint while crouched is huge, and being able to silently open locked doors is double huge.
Plague hearts aren't that hard. Bring C4 (two), thermite (five), or even frag grenades (three). Apply scentblock before you go in. Let one explosive finish doing its job before using the next one. If using C4, you need to let the heart calm down after the first one before you go in and plant the second one. If using thermite, let it burn away. Followers will usually try to melee the heart, so don't bring a follower from your community. If you really want a follower, recruit one from one of your friendly enclaves for 100 influence. (That also applies to when you're attacking hostile enclaves.)
"Sieges": if you're out doing something and don't want to come home when a zombie attack on your base is announced, go to an outpost and switch characters. The attack will then play out without you and you won't be charged the extra ammo cost.
Quests: Some of the quests are quite helpful, while others are a waste of time or too risky. Here's a quick rundown:
Aunt Prepper - Absolutely do this one at least to the point where you receive the 10/22 carbine. After that, it's still pretty helpful.
Hunt for Vinyl - This is a relatively quick and easy way to get an ally who becomes an influence farm. Do it.
Eagle Eye - follow it to get the .50-caliber rifle. I was awarded a rooftop sniper radio command once after completing this quest. I don't know how consistent that is, but it's worth a roll of the dice, because that radio command is a game-changer.
Ancestral mace - It's a fairly easy quest, but the reward is marginal. That ancestral mace is heavy, and the other weapons aren't especially good. Your choice.
Action Hero - other than the car you get when you start it (which has a small trunk), all your hard work gets you jack squat. In nightmare, the car has only a little gas and it's already pretty beaten up. I don't do this one.
Gentleman Cadaver - too much trouble for too little reward. Pass.
Chief Jackson - it's three scavenging runs that get you an unremarkable blunt weapon, a car with a small trunk, and a loud pistol. Pass.
Builder - the "elbow room" mission has been very glitchy for me, It often leads to me doing a Save&Exit/Restart to get out of it. But if it works, it's OK to do. The other builder mission gives you four materials rucksacks in one building. Heck yeah to that.
Trader - the first trader mission gets you an easy friendly enclave, so do it. This is an easy quest.
Warlord - as far as I know, the "To Catch a Thief" mission can always be resolved via intimidation, so you get a rucksack for a small amount of work.
Sheriff - nah. You're putting your neck on the line for no good reason.
The distillers - completing this one gives you a pretty good radio command (food + molotovs + luxury) and if you need a chemist to recruit, they have one. Just know that the last mission is a fight with hostiles. I think it's still worth it.
The doctors - this quest includes combat with hostiles and finding a portable generator. The reward is either a healing radio command or a medic to recruit to your community. On the whole, it's kind of a lot of hassle unless you're desperate to recruit a medic.
The mechanics - I was never able to complete this one in dread/nightmare, for various reasons. I do know that if you need a mechanic to recruit, they have one.
The cannibals - this never ends well. Pass.
Demand for resources - this quest looks like a regular mercy mission that could result in friends or allies, but if you see the word "demand," skip it. You're pouring resources down the drain and will end up with enemies no matter what.
Plague researchers - you'll never get allies from this quest, but it's a decent way to get some useful biochem items. Your choice.
Zombie researchers - I used to get this one frequently, but haven't yet been offered it in dread/nightmare. I recall it as being pretty annoying and unhelpful in regular mode.
Allies: Just because I put this last doesn't mean it's less important. Some allies provide game-changing benefits, while others are useless. But in general, you should try to make a lot of allies. For the most part, new enclaves start out as neutral. Do one favor for them, and they become friends, and another favor turns them into allies. Occasionally, you will have to do another favor down the line to maintain the alliance, but fortunately, they aren't usually very needy.
Beside the boons they provide, allies are a great way to find new survivors for your community. Recruiting someone will break up the alliance, so you have to weigh that choice.
Anytime you want to make new allies, use the "Find other survivors" radio command. The new enclave comes in as friends, so one favor for them turns them into allies. another great thing about making friends over the radio is that they usually spawn close by your base.
For me, the best alliance perks are Rooftop Sniper (radio command), Neighborhood Watch (threat reduction), and Roadside Assistance (gas & toolkit delivery). I won't break up an enclave providing these perks no matter what, and will do anything to maintain the alliance. Allies that provide things like morale boosts and parts income are nice, but if they have a survivor I want, I'm recruiting her. Then I'll call in for a new group and see what perk they give me. Allies that provide offsite meds storage or some other BS just piss me off. I'll recruit someone from their group and then immediately expel her ass just to break them up and get them out of my game.
I told you it was long.
DREAD/NIGHTMARE LEGACIES GUIDE
The quickest way to earn this achievement is to do four dread playthroughs, but I assume you also want the Over the Rainbow and Peerless achievements, which require that one of your playthroughs be in Nightmare. So, to unlock all three achievements in the minimum four playthroughs, I recommend:
Dread - Builder
Dread - Warlord with Builder legacy card
Dread - Sheriff with Builder and Warlord legacy cards
Nightmare - Trader
If you simply want to earn Tried and Tested as quickly as possible, without regard for Over the Rainbow and Peerless, I would alter it to:
Dread - Builder
Dread - Trader with Builder legacy card
Dread - Warlord with Builder and Trader legacy cards
Dread - Sheriff with Builder and Trader legacy cards
The builder boon is far and away the best, which is why you want to get that legacy done first. It only has one required fight with hostiles, which is in your base and is at the very end of the game. Choose your three carryover survivors and equip them for your next game before you begin the builder's final mission.
The warlord has a lot of chances to get into hostilities. He can usually threaten his way through them, but sometimes will have to fight no matter what. His fights are away from the base. He should recruit a friendly follower for 100 influence to take on his missions instead of bringing one of his own people. Once again, choose your three carryover survivors and equipment for your next game before you begin the final mission, which is called Smash the Coalition!
The sheriff's ending is similar to the warlord's, but I think more of his missions end up in hostilities. Use the same strategy as the warlord - take a friend (not a community member) and choose and equip your three carryover survivors first.
The trader's ending is similar to the warlord and the sheriff's, but she only has to fight hostiles once, and there is only one specific enemy that has to be killed for her to win. Her final mission is called The Summit.
The carryovers you should try to bring with previous community members are:
Skills - computers, gardening, utilities, recycling, mechanics, chemistry, medicine, soundproofing.
Weapons - 10/22 carbine, Echo X-3 crossbow, .50-caliber rifle, your favorite bladed and blunt weapons.
Rucksacks - always bring 3 rucksacks of materials!
Items - Red Talon Workshop prefab, .22 ammo, light crossbow bolts, .50-caliber ammo, C4, thermite grenades, skill books, network signal booster, water cooler (if no builder card), portable generator (with or without builder card - this is a frequent ally request), CNC mill, handgun ammo press, fertilizer, toolkits, vehicle upgrade kits, 1 gas can, plague samples, parts, scraps of circuitry, blood plague bulk pack, other luxury/influence items.