Phantom Dancer Urgot / Not Functioning.

So i was looking at the new phantom dancer item. And I saw its passive effect Lament which reduces the damage of a champion who was last hit by you by 12%. So i was thinking. That can sound amazing with Urgot. Urgot, whose passive ability is to reduce damage by 15% for 2.5 seconds using basic attacks or seekers. SO i combined them and it SHOULD reduce the damage done by an enemy by 27%. HOWEVER i am still in lane getting CRUSHED by 2 spells.

When I ult that is another 40% reduction of damage to me. How am I STILL getting MELTED when I have 67 FUCKING % DAMAGE REDUCTION on an enemy? That makes 0 damn sense. Image i through EXHAUST into the mix. ANOTHER 40% reduction. I GUARANTEE that even with 107% damage reduction i still would get melted. Which makes 0 sense. If i resist 100% damage i should be taking 0. Not to mention mr armor and magic resist. Meaning that FURTHER reduces damage by a percentage.

So what is going on? Why is it that these combinations are not working? Why would you put all this damage reduction but when I actually get hit I go down in 1 combo after debuffing them for 67% damage reduction?

Explain this shit.