This is how you make thornmail work

You don't stack armor neccesarily although some armor is needed 200 bonus armor including thornmails should do Any armor beyond that isnt needed.

Then what you want is all the % damage masteries so just 18/12/0 Since tank masteries are useless anyway

grab the % increases everywhere and grab the magicpen

And then grab fervor because fervor is op and doubles your basic attack dps as a tank, you will need about 10k health to make grasp compare so just take fervor

And then you buy this baby