Some more zed fighting tips.
I used to hate Ferals the most, now Big 'Uns have moved up to my most hated status. A Big 'Un can rip a completely healthy character in half if no one is there to knock him upside the head to make him drop you. I lost one of my favorite heroes to a Big 'Un exactly like this during a besieged event where one plucked me out of the house because I was standing too close to the front door while fighting zombies. The three nimrods I was trying to protect happily watched me get ripped in half just a few feet away but apparently walking from the comfort of their house to their front porch was too much to ask. The point I'm getting at is be extremely cautious of the big guys. One error can be fatal, especially if you are fighting one by yourself. There is an option to take a buddy with you everywhere you go (at the cost of 100 influence) but I'm always hesitant to do this because they tend to do things like run at hordes you just chucked a firebomb or grenade at and then be upset because they are on fire or whatever. Pussies.
Ferals are still a handful but you at least have a chance to recover from serious wounds once or twice while fighting them. The biggest risk while fighting either a feral or Big Un is having other zombies around to distract your focus and give the freaks an opening to maul you. Always try to have a vehicle close at hand to use for a hasty retreat if you need it.
Know your terrain. One aspect of SoD that irks me is that zombie density/difficulty is completely opposite of logical sense but I think it has to do with the AI pathing. When you are in one of the cities, zombies primarily travel along the roadways and they aren't likely to leave buildings unless you approach very close, drive a vehicle nearby or shoot a gun off in close proximity. Even zombies relatively nearby won't always run around buildings to get to you even if they know you are there. This allows you to slowly but systematically clear a block or a row of office buildings while dealing with bite size groups that can be dispatched semi-easily. When you are out in the country things are actually much worse. Without rows of buildings to camp in or roads to influence travel direction zombies will flock in from ridiculously far away at the slightest provocation. You will rarely have a simple encounter with just a few zombies out in the country, instead more and more will trickle in as they are attracted to the commotion, travelling in fast straight lines directly towards you. The majority of my worst maulings and encounters that quickly go south have been out in the country fields or lonely farmhouse areas of the map. My first Breakdown attempt I had my home base at MacReady's farm and was constantly under attack from zombies wandering in from the countryside as well as victim to orchards directly adjacent to the farm that were always infested (I don't know if it changed or not since I rage quite that doomed attempt but I couldn't build outposts in the orchards to prevent the infestations because you need to be in a building to create an outpost.)
Silence is golden. Zombies in this game are extremely sound orientated. As such, silencing your firearm is very important. Silencers come in three grades. Super cheap tin can models good for ten shots, better homemade ones that will do thirty rounds and very rare machined silencers that will last for fifty. I not only recommend having at least one firearm with you for when things get sticky, but also have a silencer for it. One of the most difficult hero challenges I've done so far is killing fifty zombies with revolvers. The revolvers only hold five shots a piece, are slow to reload and can't be silenced. Not fun.
Variety is the spice of life. To get the various challenges completed you want to have a diverse group of skillsets availabe. Shooters, edged weapon experts, blunt weapon experts, heavy weapon experts, etc. The same goes for traits for optimizing your base. Have a researcher, a medical guru, a construction expert, you get the idea. Choose your specializations wisely. I talked up rage early on but after messing with it I decided I like combat endurance better (50% reduction in melee stamina use.) Having at least one ninja around is also handy. The point here is don't build all your characters to be clones because if you run into a situation they aren't suited for you won't have a plan B.
A few general Breakdown tips. Fix the RV as fast as possible even if you plan to stick around a while, otherwise you might not have the materials available when you do finally decide you want to leave.
You can only take seven survivors with you to the next level, choose wisely. Speaking of choosing wisely, all of your gear in the storage locker comes with, but make sure if someone you plan to leave behind had a good item, you have them store it before you leave or it's lost.
Item, material and influence management are keys to success, especially when you first arrive at the next level. When you start out influence management is especially vital to survival. You'll be burning through influence to set up outposts, and even more importantly, restocking with choice gear out of the storage locker. There is a hard limit to how much you can stow in the locker so before you swap levels destroy the useless crap in the locker like small backpacks and 2x4's. This frees up room so you can store useless crap in the locker again to build up influence fast at the start of the next level. You can also carry over up to 30 of each resource type from the previous level, if at all possible try to carry over the full 30 building materials so you can move into a home site ASAP if needed. The RV's placement in the valley when you start a new level is random. Sometimes you can put an outpost right next to it to help shield your people from wandering hordes with the outpost traps, sometimes you can't. And on the later levels even that may not be effective for long because the zombie attacks become relentless. Also be mindful of resource storage caps, don't bring back resources you don't have room to store, they just end up wasted and I cannot stress how finite they become later on.
Get your researching done in the first level or two. You will need to do a lot of building, tearing down and building more to get all eight of the research types done (and you want them, they give you access to create your own gadgets which becomes much preferable to relying on the every stingier game world as you progress) but availability of building materials drops off drastically after the first couple levels. Also, research knowledge carries over each level so once you learn how to make chemical incendiaries, you will always know how to make them.