I disagree with the answer posted by ElDuderino. Viability is important, and it does not simply mean that the build at hand can do those things deathless & with minimum time spent.
In fact, a build can be endgame-viable, yet still fail to do content reliably deathless.
Let's say you're playing an ignite build. Most of your damage (~90%) comes from igniting your target. Now you try to fight Atziri / Uber Atziri, who is immune to ignite. You're fighting her on 10% of your usual damage, and will most likely fail killing her, and even if you manage to kill her it will require high-end-gear as well as a lot of skill, luck & time.
So, builds relying solely on ignite damage are not viable for something like Atziri.
Another example: Guardians. With the "Atlas of the Worlds"-Expansion new endgame content was released, including 5 new maps. 4 of those contain the guardians, powerful bosses which have to be slain in order to acquire fragments. These fragments are then used to enter the Map where "The Shaper", the endgame boss, resides. The guardians all have immunities. The Phoenix for example is immune to being ignited, the Hydra is immune to being poisoned, etc.
Last example: Let's assume you're playing a glasscannon build. You skip all the life nodes on the passive skill tree, and only take damage, more and more and more. At the end, you might be sitting on 2k life. Now you attempt doing the uber-lab. Even normal monsters will likely oneshot you.
So, concluding this answer, no, it's not about doing something deathless and in the minimum amount of time. It's about being able to do something reliably, with a good chance of success.
Now, to cover a few basic things regarding viability. Please note that this segment will only cover basic beginners knowledge, mostly regarding survivability (stats that (basically) are required in order to make a build work).
Lifepoints: If you're playing a life-based build, you should opt for having around 4k lifepoints in the early lategame (while doing maps up to T10). When starting to grind red maps (T11+) you should try to push for 5k, optimally 6k+.
Armor/Evasion: Armor reduces the physical damage taken, based on the amount of armor and your level, meaning that 100 armor are very good at level 1, but very bad at level 70, generally speaking. There are quite a few builds that almost completely ignore armor, and focus on evasion rating, which increases the chance of the enemies missing.
Both of those choices have pros and cons. There are map mods which reduce the players armor by 40% - 60%, and there are map mods which increase the monsters accuracy by 40% - 60%. Both of those map mods can be quite detrimental, depending on which defensive setup was chosen.
I cannot name an exact amount, but if you're going for an armor build, 60% physical damage mitigation should be what you're aiming for.
Resistances: Path of Exile has 4 different resistances. The elemental resistances (Fire/Cold/Lightning) and chaos resistance. These are capped at 75% (meaning that if you have 120% lightning resistance (after subtractions) the game will count it as 75%).
The following paragraph is subject to change in the 3.0.0 release, where difficulties will be removed entirely, and the penalties will occur at certain points in the storyline.
Subtractions change with the difficulty. In normal difficulty you'll not have any subtractions, in cruel you'll lose 20% chaos resistance (so you're at effectively -20% if you have no chaos resistance anywhere), and in merciless you'll lose 60% for every resistance.
Keeping your elemental resistances capped is absolutely necessary. Without 75% resistance to elemental damage, you'll most likely get oneshot very quickly, even when you're at 6k life.
Chaos resistance is not that important, except for content like "Chayula", since that boss deals only chaos damage, and a lot of that. So, chaos resistance, not all that important. It can, however, make certain boss fights go a lot smoother, and a few monsters do deal chaos damage as well. Here's a list of monsters & bosses that deal chaos damage.
So, the stats you want to have when transitioning from early lategame to mid lategame, purely focused on survivability, are atleast 4k lifepoints and all elemental resistances capped!
Attributes: This is a bit confusing at first, but I hope it's understandable. When you're playing a caster for example, you're going to naturally be looking for intelligence as stat on your gear, since it increases your mana pool and your energy shield. That's something you'll see in most games, but for PoE, you have to adapt a bit. Due to the nature of the passive skill tree that PoE has, you'll get a ton of intelligence by traversing that skill tree. A lot of skillgems that are used in caster setups do not only require intelligence, but also or only dexterity, or even strength, meaning that having dexterity instead of intelligence on your amulet can be very beneficial.
Rule of thumb (for me) is to keep my main stat as high as possible (strength in my case), while not letting the other two (for me that's intelligence and dexterity) fall below 120, since I do need those for my skill gem setups.
Tooltip-DPS: A common beginners mistake is focusing too much on the tooltip-DPS. There are a lot of builds where the tooltip-DPS is simply misleading, for example builds that focus on DoT (Ignite, Poison, Bleed). The skills used to apply those DoTs usually have a very low DPS (like 2k), while the DoTs they apply deal a disgusting amount of damage, that is not properly displayed in the tooltip.
So, don't rely on the tooltip-DPS too much, it only leads to confusion. When you're playing a raw damage-build however, it's pretty accurate. In this case I'd aim for ~12-15k, that should be sufficient for everything up to T10 maps.
The beautiful thing about Path of Exile is that every build has advantages and drawbacks.
A pure elemental damage build can focus on stacking %-increased Damage nodes for their elemental type, but it cannot deal with the map mod "Monsters reflect X% of the elemental damage taken" (most likely even with Vaal Pact), since you most likely will just oneshot yourself.
A pure physical damage build can focus on stacking %-increased Physical Damage nodes, but it - same as the elemental damage build - cannot deal with the map mod "Monsters reflect X% of the physical damage taken".
What can deal with reflect-mods is a build where you're not the one taking the reflect damage, but instead your minions or totems. Those builds have the drawback of usually being quite expensive due to the items required.
Generally speaking, a beginner should look for a build labeled as "Beginners Build". These are usually easy (mechanically speaking) as well as cheap. One should also check if the guide has been posted/updated recently, as the functionality and viability of builds can be heavily impacted by a small patch already.
You should also not try to get a beginners build going which is cheap, easy, and fully lategame-viable, that most likely will not work out. What most people - including me - do (per league) is creating a character using a cheap build that can tackle up to T10 maps with ease, and then use that character to farm enough currency/equipment in order to be able to equip a character that's fit for lategame.