At its most base level, a fight is a race to eliminate health bars. The person who still has health left in the end, wins. Sounds simple. There are many nuances to what it means and how fights are won, but this is the essence of a fight.
There are really only two components in this race: the rate you do damage and the rate you take damage.
You have a finite amount of health, and damage mitigation is simply reducing the rate at which you are hurt. There are multiple ways that you can reduce the rate you take damage.
Everybody has a different amount of health, and we don’t care so much about how many points of damage you take but the percentage of your health you lose per second. Having more heath means that your opponent will do a smaller percentage of your total health in a given hit.
Unfortunately, health is an expensive stat that costs roughly twice as much as armor or magic resist. For a team, this is one of the least efficient means of winning a fight.
A single point of armor or magic will increase your effective health by 1% against a specific type of damage. This is amazingly powerful, but has two nuances: 1. you need to have health and 2. it is only good against a specific type of damage. If you are a tank, the general wisdom is to spend half of your month on health, then a quarter on magic resist and a quarter on armor. This is the most cost effective means of amplifying your health to maximum potential, but it is predicated on having health which is expensive. As a means of gold spent to mitigate damage, it is still not the most effective for a team.
You take zero damage per second for each second an opponent can’t do damage. Sounds like crazy simple math. Here is what is interesting about crowd control in League of Legends: it doesn’t scale with gold. Yes, you can buy cool down reduction to do it more often, but most engagements are so short that you can rarely use crowd control more than once in a fight. This is one of the primary reasons that supports are usually champions with a huge amount of crowd control abilities — because you don’t have to give them gold and yet they can help mitigate a tremendous amount of damage by using their abilities.
Many of the most potent crowd control abilities, unfortunately, require that you put yourself in harms way by being in close range. This often means that if you fail to lock up your opponents, you may need other forms of mitigation to escape the opposing team’s damage. (Like health, armor, or magic resist.)
A second issue with crowd control is that it lasts for a limited duration. No amount of cool downs will generally allow you to lock down opponents indefinitely. You might be able to stack crowd control with other people on your team, but eventually it will wear off and to maximize your use of it, your team must be doing damage in this time.
In the race of who is doing damage faster, this is one of the most cost effective ways of reducing an opponent’s rate of damage to zero. Not just some percentage of damage, but no damage at all.
I just finished saying how great crowd control is and how cost effective it is. There is a second really amazing form of damage reduction. An opponent does no damage while not taking shots, of course, but you can’t control what shots they take beyond staying out of range: so the next best thing is they do no damage for every shot they miss.
Positioning is aided by movement speed and abilities that allow repositioning or give you extra range. It requires knowledge of opposing champions, their range, and the ability to anticipate their attempts to do damage so you can avoid them.
If you have longer range, you can also do damage without them being able to do damage back. You simply have to make sure to maintain that range as they attempt to come your direction.
Positioning tends to be dirt cheap (boots or cool down items to activate dash-like abilities), but takes a tremendous amount of skill and timing.
Stealing life and healing is not damage mitigation directly, but it reduces the rate at which your health bar is going down per second and that’s a key part of the race. Many effective damage items happen to also offer some form of life gain.
Summary of Damage Mitigation
The cheapest and most cost effective damage mitigation is positioning and crowd control. They cost virtually nothing or very little, and allow you to take little or no damage at all. Life gain is often done through abilities or through items that also do damage.
There are three types of damage in a fight: burst, area of effect, and steady. Through the use of items or abilities, some champions have the ability to quickly do huge amounts of damage to a single target. Area of effect damage is merely damage that impacts multiple targets and has a very high “damage per second”, but that damage is spread out over many targets and is unfocussed. Steady damage is the damage done by auto attacks and has very few spikes (with Critical hits being the only real spike damage and by late game you’ll critically hit with most shots if this is the type of champion you are playing.) Steady damage can by high, but may give opponents time to react so they can either use crowd control or burst you down.
All three types of damage require items — either to reduce cool downs or to increase ability power, critical hit chance/damage, attack speed, or attack damage.
The goal of area of effect champions is to soften up targets before an engagement and create a favorable fight. The goal of burst damage is, once the fight has started, to spend all of your cool-downs quickly and eliminate your opposing steady damage dealers, while your steady damage dealers wear down their more durable champions that a burst damage dealer would not be able to take out.
Damage requires items and damage mitigation can be done with fewer or no items. So, the general wisdom is to first put money into the damage dealers, as defensive stats are generally about surviving engagements, not winning them.