Is Void Staff the only option then? That's pretty lame. What if after Void Staff you still aren't doing as much damage as you'd like? Just pump at that point?

Since flat is calculated first, then % based... If you bring them below 0 with this, what happens then? Or can they not be brought below 0?

Mastery for 15% mr reduction, void staff for 40%, sorc boots for 20 flat should take care of the problem. Guise would give another 20 flat

Penetration can't knock them under 0, only reduction (amumu/fiddle passive, karthus wall, abyssal scepter, etc)

and reduction calculates before penetration

if they're under 0, reduction, both flat and % do nothing

The way MR is calculated is:
Your magic resistance divided by your magic resistance plus 100. In numbers
MR
MR+100

Starting out most champs have about 25% reduction (your bombs were doing lets say 500 damage now they do 375 damage) this almost never increases without times. Buying a Negatron Cloak will put you at about 50% reduction (bombs now do 50% damage)

To counter:
Void Staff - This penetrates 40% of their MR Meaning that if they have 200 MR (67% reduction) you would deal 13% more damage (reduces MR by 80)
Sorcerer's Shoes/Haunting Guise: these penetrate MR by a flat amount (20) meaning that they are more useful if the enemy has low MR you will deal more damage (about 15% if they dont have any items) however if they have 200 MR you will deal only 3% more rather than the 13% with a Void Staff
Abyssal Scepter: REDUCES not penetrates MR by 20 this isnt really any different from penetration, because you cannot have more than one of the Auras applied. However with certain champions which have skills that reduce MR it is quite effective (for example Fiddle's Innate aura reduces their MR by 16 i think and this by 20 which mean they have lets say -6 MR this means you deal BONUS DAMAGE this does not stack with penetration, i think, and it is applied before Voidstaff's 40%)

If they are stacking a lot, grab Voidstaff, if not really grab sorc boots, which you should always have on zil

Math is off.

If Amumu stacks 100 MR in addition to his current stat of 100/(100+100)
it would be (100+100)/(100+100+100) or 200/300 = 66%

Quote:

Originally Posted by Zack1996 If your Zilean build doesn't include Void Staff, the current metagame will eat you alive. And the amount of damage you take from stuff is: 100 / (100 + MR) So with 100 MR, Amumu would be taking 100 / 200 damage, 50%. If he proceeds to stack 100 more MR, it'll be 100/300 damage, 33%. Which effectively means that: a) You need to deal 2x the normal amount of damage to kill him, when he has 100 MR. b) With Void Staff, this amount is reduced. eg. 100/ (100 + 0.6 x 100) = 100/160, which is 0.625%. Therefore, you only need to do 1.6x of the normal damage. Effectively, you need to do 80% of the amount of damage required without Void Staff, so your damage is effectively increased by 25%. The more MR he stacks, the more effective Void Staff is.