4 item tanks beats 6 item marksmen in a duel via THORNS ALONE. Mathematical proof:

marksmen items:

There are very very few marksmen that can acquire these stats with any combination of 6 items, but for the benefit of the doubt lets say that this is the stats the marksmen has at 6 items:

1k crits

100% crit chance

2 attacks per second

CARRY:

2k dps

35% armor pen

Tank items:

TANK: http://leagueoflegends.wikia.com/wiki/List_of_champions%27_armor

assuming 75 base armor at level 18

take +100 from thornmail

+95 from frozen heart

+75 from randuins omen

+25 from ninja tabi

+13 from armor runes

+5 from masteries

+4-20 from masteries

multiply all bonus armor by 1.05 because of masteries

that makes 408 armor, with only 4 items. You could have 2 magic resist items. Like spirit visage and malmortis for example.

Lets just round down.

400 armor

72% damage reduction after armor penetration

32% damage reduction from auto attacks by attack speed slow from frozen heart and randuins omen

10% damage reduction from auto attacks by ninja tabi

83% total damage reduction

So the tank takes 343 damage per second. Thornmail alone would do 400 to the carry if the carry has 50 magic resist. 30% of 2000 is 600, and 50 magic resist is 33% damage reduction. And no you aren't going to lifesteal through it-- imagine what you would have to have to lifesteal through it. You only deal 343 damage per second after armor and are looking to restore 400 health per second? You'd have to have more than 100% lifesteal to cancel it because thorns is doing more damage to you than you are doing to the tank. In fact, if you have less than half the health of the tank and less than 50% lifesteal, then in a straight duel the tank can kill you without touching you, just from thorns.

Proof: Thorns returns 400 dps You deal 343 dps You restore 171.5 health per second

Each second you take a net 400-171.5 = 228.5 damage per second The tank takes 343 damage per second

In 10 seconds you will take 2285 total damage even accounting for lifesteal and magic resist. In 10 seconds you will deal 3430 total damage to the tank, accounting for all armor and attack speed slows and such.

So basically, if you have less than half the health of the tank and you begin a straight duel without interference, you will kill yourself before you kill the tank. If the tank manages to hit you with any spells whatsoever, it pushes it into his advantage that much more. Bruisers who build this way and also have tremendous base damage or % health damage end up being a serious problem because they can be a threat to your team without building any offensive items. This is the reason Mundo, Shyvanna, Renekton, Nasus, and friends are so difficult to deal with.