How do you counter thronmail

MR and more lifesteal.

why do u focus a target that's thornmail heavy? no other avaible targets? last whisper/lifesteal

Mercurial Scimitar or Maw of Malmortius plus life steal. (I'm assuming you're an ADC) Elixir of Wraith consumable will inherently negate 33% of the thornmails potential damage before magic resistances on its 10% steal alone.

Between BT and Elixir of Wraith you're already returning 30% with no resistances which should more than counter thornmail, magic resistances never hurt though.

An ADC with a decent build wont be physically hurt by a thornmail but instead their level of sustain/recovery will be hurt which when there is no escape is the only thing they have to fall back on.

MR + Armor Pen + Lifesteal

Spirit visage gives you almost everything you need. MR, CDR (so you can do some spell damage), and increased healing. Add a lifesteal item, and now it's countered. Also bork is better than bloodthirster because on-hit effect damage isn't reflected, but applies lifesteal.

If you're the main ad damage dealer dont focus the tank (they will most likely be the ones with it)... focus the back line and let the ap deal with the thornmails...

if your in a situation where you must attack thornmails, build alot of penetration and lifesteal ...last whisper black cleaver(if it fits your build) , bloodthirster etc...

it will allow you to kill them with the least auto attacks(because of the armor pen) which will lessen the damage recived by the passive where as the lifesteal will help replace the health lost from the thornmail which kind of cancels out the passive of the thornmail

Use your skills instead of auto attacks.

if youre an adc you need to stack lifesteal so you heal up, as much as, or more than thornmail is hurting you.

totally ignores .

Thornmail is a counter item itself. While there are ways to lessen what it does to you, you don't need a counter for a counter.