It's been quite awhile since I subjected you guys to one of my mechanics lectures, so I thought I'd inflict some pain.
Let's talk about tanking. We'll call this lecture "Tanking 2.0".
I'm going to teach some more advanced techniques in itemization for any sort of tanking. And yes, tanking is 90% itemization. This is why you need to learn how to be cost efficient when it comes to tanking, because items means everything for a tank. So if you ever wondered why some tanks are invulnerable and why some are so fragile, I'll teach you. Oh yeah, and these are advanced tips, so I won't cover what exactly resistances and health are. Only more obscure stuff.
1. What exactly are you defending against?This is important. A tank can't just buy items to protect himself if he's not going to actually be using them after all. The nature of this question is very different depending on your champion. For example, Garen will almost never have to buy MR and indeed, he almost never does. Even if the enemy has three AP casters, Garen will probably still do fine getting health and armour. This is quite obvious if you think about it, what kind of mage is going to waste his spells on Garen? Similarly, Kassadin almost never buys armour, as he's not going to be taking much damage from sustained AD since he's all about appearing suddenly, unleashing all his spells, and running away. It wouldn't matter if the entire enemy team was AD.
So, if we elaborate on this, building defensively doesn't blindly buying items just based on the enemy team. It means buying items based on what kind of damage the champion specifically is going to be taking. There is no easy way to learn this, it comes with experience and identifying what your champion is explicitly dying to most of the time. I'll give a very simplified version however:
- Tanks die to sustained damage
- Squishies die to burst damage
2. What is burst damage and why do I need to care?Burst damage is the single largest source of pain in the game. A huge majority of games end before sustained damage even becomes a factor. It's an abstract concept. Basically, if an enemy can inflict crazy damage on you, but can only do it once in awhile, it's burst damage. This can be both AD or AP. Examples of AD burst casters are Pantheon, Garen, and Talon. AP would be almost every mage and most assassins.
Right off the bat, if you're fighting a burst caster, building resistances is suboptimal unless you have ridiculous amounts of the resistance. I could math up the reason why, but I doubt anyone wants to read through paragraphs of math. So if for example, you're fighting Akali, having 50MR won't save you in the least. Conversely, if you're fighting Garen and you have 200 armour, then you are in a very good position. It's generally much easier to stack armour than MR, as armour is cheap and plentiful. However! There is one supreme stat that rules over all burst damage!
Health.
Generally, if you're sent up against a lot of burst damage. Stack health. Stacking lots of flat HP means they will never kill you and that's all that matters against them. When was the last time you saw a Pantheon singlehandedly kill someone with Warmogs? Or Morgana? It's impossible. It'd take them minutes to kill someone with 3600hp.
This is why getting HP is the best choice if you're a champion that only fears burst damage. This is why Veigar works well with Warmogs and Kassadin almost always wants a RoA. They like the health. It's also one of the reasons why Banshee's Veil is superior to Force of Nature if you're fighting burst casters. Aside from BV's shield, FoN doesn't give any health. So an Ashe with FoN will still die easily to an assassin.
3. What is sustained damage and why do I need to care?Sustained damage is what defines late game. It's why most mages fall off in very long games and why carries exist. It's what its name implies, it's reliable, neverending damage. Basically, if your champion isn't going "I'm useless!!!" for half a fight, then he does sustained damage. This does not include just carries though, but most tanks and supports are sustained damage dealers too. The magnitude of the damage is different, but the way to defend against it is the same.
Stack resistances.
Simply put, resistances give you waaaay more bang for your buck than just getting flat HP. Mathematically speaking, you get far more effective HP from stacking resistances than you would buy stacking health. The only reason why this isn't the case for burst damage is because:
1. Getting lots of resistances when you have no HP doesn't help when the enemy is able to output 2000 - 3000 damage in a few seconds. You'll still die before you're able to react. E.g: Ashe with a FoN will still die to Morgana while Ashe with Warmogs will not.
2. Most burst damage is AP, and stacking MR is a lot harder than stacking HP. This is how the game was designed. MR items tend to be specialized, expensive, and in pretty small amounts (Thornmail and Frozen Heart give 100 armour, FoN gives 76). This is primarily why AD burst casters become useless in the lategame. Armour is too cheap. It's also why casters are so strong early on. Everyone's HP is so low.
However, when you're going to be subjected to constant, (relatively) small, and unremitting damage, then logically you're going to want the option that defends you the most right? This is why you buy resistances against the likes of ranged AD, Swain, Vlad, etc.
4. Against Mages: Health vs MRThis is a continuation of sustained vs burst damage. Have you noticed that MR tends to be expensive and hard to get? This is by design because unlike AD damage, almost all AP damage is burst. So if you read the above, you know that burst is hardcountered by stacking health. In other words, there are actually two very efficient ways to counter AP damage (yet another reason why most mages tend to fall off as the game goes on).
To give a simple example: A Mundo with a Thornmail and a Warmogs will do perfectly fine when fighting LeBonk but a Mundo with a FoN and a Warmogs will get stomped on by Graves.
This is why, generally speaking, if you have lots of HP, you won't need to buy a lot of MR. And since HP also works against AD damage, you almost always want to buy HP instead of MR. You might have noticed that as a player you've been doing this innately without realizing it; you rarely ever buy an MR item because it has MR. Most people I know buy an MR item because it does something else while conveniently giving MR. This is very different from armour or HP, where people explicitly go out of their way to get an item specifically because it has that stat.
In practical terms, if you're only fighting one or two sources of AP damage, then just buy HP. There's no need to explicitly go our of your way to buy MR. But if you're fighting a lot of AP (3- 4), then you're going to want to consider getting MR as well. However, this is only true if you're fighting a bunch of burst casters. Against sustained AP, you buy MR no matter what.
5. Are you dying to damage or to CC?A lot of people don't realize that most deaths could have been avoided with proper positioning and timing, and that all the defensive stats in the world couldn't have saved them. An Ashe with 200 MR would still die instantly to chain CC from Morgana, Lux and Ryze for example. If you notice, most pros avoid tanky items almost completely, as they are confident enough in their positioning to avoid getting hit by damage in the first place.
To further illustrate the meaning behind this question. Consider the severity of an Ashe getting caught by a snare compared to a Mundo getting caught by a snare. If Mundo dies, it's quite likely that he dies to damage and not to CC as any damage that comes before the snare ends will not be enough to kill him. However, Ashe is probably dead way before the snare's duration completes. So in this case, we can say that Ashe died because of CC, not because of damage. If Ashe didn't get caught by the snare in the first place, she'd still be alive whereas if Mundo didn't get caught by the snare, he'd be in probably the same situation as always.
It is actually possible to itemize to improve your positioning and fix mistakes. Items like Banshee's Veil, Merc Treads, Trinity Force, Guardian Angel and Phantom Dancer all help with this. It all comes back to the first point, there's no need to build tanky (thus saving lots of gold) if you're not going to be recieving any damage. That's the entire idea behind this point. The strongest form of defense for a squishy is to not be in a position to take damage at all.
It's also why my Sivir just stacks Bloodthirsters unless there's lots of assassins on the enemy team.
I think that about covers advanced tanking techniques. I'll summarize.
Tl;dr:
1. Don't buy useless stats that don't help you
2. Buy health against burst damage
3. Buy resists against sustained damage
4. Health is better than MR against most mages
5. Positioning > any defensive stat