League-of-legends – Can true damage dealt be amplified by the mastery Double-Edged Sword

Okay, from my experiments, it goes:

Armor/MR reductions <--> Percentage Reductions
(They stack multiplicatively, so order is irrelevant)

Flat Reductions

This makes sense, since if flat reduction was the first thing applied, after armor, it may have failed to provide a benefit at all (since, for instance, the difference between 50 * 27% and 48 * 27% is minute at best. 50 and 27% being the base attack damage of Ezreal and the armor of Amumu, my two test characters.)

Shields act as health, and damage done to shields is likewise reduced by armor / magic resist. (Meaning shields on tanks can generally absorb more punishment than shields on squishies)

In short no. Not sure if there is a hard limit to damage reduction (like Cooldown reduction cannot exceed 40%), but damage reduction is based on a soft reset where they multiply, not add together. We'd first look at the effective armor/MR that the champ may have and look at the damage that is incoming.

Let's say you have 100 armor effective after all the armor pen, and they are doing physical damage of 100. This would mean you take 50% less physical damage. So their 100 damage would now do 50 damage to you. If you are in Maokai's ultimate, this is further reduced by 20%. 20% of 50 is 10, so it blocks 10 more damage. You would take 40 damage instead. If you have Leviathan fully stacked you take 15% less - that would be 40*.15 = 6 damage less or you take 34 damage now. If you are Poppy and were to die to this 34 damage incoming, you reduce that by another 50% and you now take 17 damage.

So effectively you end up reducing 83% of the damage that you would have taken. This is with reduction of 50% from armor, 20% from Maokai's ult, 15% from Leviathan, 50% from Poppy passive. Or we could just multiply how much you would take from each together: 0.5 * 0.8 * 0.85 * 0.5 = 0.17