How do you counter thronmail

Lifesteal - Armor Pen - Magic resist -

definitely need to stack the ar.pen and lifesteal if the enemy starts getting multiple thornmails + other armor/hp items

+ MR item, preferably or if u want damage, if not, get spirit visage or banshees

MR actually counters thornmail MUCH more than lifesteal

Last Whisper, attacking that person only when absolutely necessary, or relying on abilities.

Let the APC handle him.

(what's that? you don't have an APC?) put your hands between you knees, and kiss you butt goodbye.

Lifesteal and Magic resist

Poke with your spells, it only counters AA.