well, against most tanks, sustain is their greatest weakness because they can't outdo it. their hp and damage have a limit
for instance, there has been a teemo build going around that recovers hp as fast as chogath can delete it, if you believe that.
then champs like Vayne come into play. I've never seen a vayne player use runaan's but they should with the range increase and additional targets
she is so short ranged that tanks can just position her for their own squishy or delete champ to take her out but with runaans, she hits that 3rd proc often on multiple targets
Blade OF THE RUINED KING - this, it hits tanks very, very hard
kogmaw: a tank's worst enemy...he shreds their stats and their hp and can outrange all of them. AP kogmaw = up to 15% of the enemy's hp in magic damage per auto attack (10% if you build attack speed)
but again, I mention champs who can outsustain them in a fight. Fiora, master yi, that life steal teemo, anyone care to mention others?