Hp,mana,mr, and armor scalings
characters like dr.mundo have HP scaling on there abilities so he does HP damage on his abilities and its represented as AP so I'm wondering if it is stopped by mr or no
3 Comments
AP/AD has nothing to do with it.
MR reduces magic damage, Armor reduces physical damage.
There are some oddities, like Corki doing magic damage with AD ratios, but mostly the two match up with their respective types.
It usually says what type of damage it does Like it'll say Mundo's E deals physical damage, or that the Titanic Hydra deals physical damage, or Shen's taunt deals magic damage
it doesnt matter what the ability scales on. What matters is whether its dealing physical or magic damage. Those are 2 different things.
For example, some of 
's abilities scale off of both AD and AP, but they will always deal magic damage. So you'd build magic resist.
Something like 
's passive, will scale off of AP, and deal % of your health in magic damage. So you'd want magic resist, but not necessarily health.
Mundo's Q, deals a percent of your HP in magic damage. This specific ability scales off of nothing but your HP. So you'd want magic resist, but not necessarily health.
His E however, scales off of his bonus health. It makes his next auto deal bonus physical damage. So you'd want to build armor and health.
When I was learning league, I just looked up whatever champion just destroyed me and tried to understand them a bit better.
In the end, what you actually build depends on who you are and the team you're up against. You might not want to build 100% to counter mundo. He isnt going to be doing too much damage quickly, and their adc or midlaner might be more threatening.