Randuin's has always been a better counter to Tryn than thornmail. Why? Thornmail deals damage. Tryn's ult makes damage irrelevant, and he's at his strongest at 1 HP. Also, armor pen greatly reduces the effectiveness of Thornmail, but not the AS and Speed slow from Randuin's.
Originally Posted by RedSerpent
Get 2 exhausts, now the game is a 2v3
Whoa! A time traveller! Welcome to the future. Let me update you on a few things. In your time period, Improved Exhaust (Cripple mastery) made Exhaust a 3.5 second 100% blind + slow. That's 350% total AA reduction over it's duration, and 3.5 seconds of slow. Here in the future (which we call the present), Exhaust is a 2.5 second 30% damage reduction and 50% as slow. That works out to a 65% total reduction in AA damage per second, or a total AA damage reduction of 162.5% over it's duration and a 2.5 second slow. It's not the AA nullifier it once was.
BTW, using two summoner spells to negate one champion isn't exactly a fair situation. What if you get double exhausted by their team as well? I don't know many AD champs that thrive while double exhausted. It's not specific to Tryndamere.