Isn't the best way to handle tanks as Rengo to delete the other teams carries in .3 seconds, as Rengar is want to do, then let your team clean them up?
Assassins aren't really supposed to delete the tanks (or win against them generally, you know Burst>sustained>tankiness>burst and all that), and honestly with 3 of the 4 armor pen items in the game in your kit, I feel like unless the tank itemized SPECIFICALLY to fuck you over (by which I mean stacked armor and threw MR out the window), you should probably be able to hold your own.
UNLESS you mean Garen, Skarner, and Mordekaiser, in which case counterplay for them is pretty much the same as anyone else, clench your butthole and try to kite them to your team before they assblast you.
Also, some specifics might help. Do you have trouble ganking the tanks if they rushed an armor item? are you incapable of deuling them if, say you ran across one alone? are they blowing you up in teamfights? a very general "how do I handle tanks" is hard to answer.