I killed a Tristana without touching them as a Sion with gromp buff/thornmail

For the maths:

{quoted}

If an ADC has both BT and BotRK, he'll either lack IE or a dedicated MR item.

Let's go with the lack of dedicated MR item first: 30 MR + 27 from runes (assuming all per-level MR) = 57 MR for our carry. With a dedicated MR item like Banshee's, that would be 112 MR.

Target has, let's say, 150 Armor after penetration (which is feasible between Thornmail, base Armor and another big Armor item like Randuin's).

The actual damage each auto does is irrelevant, as both Lifesteal and Thornmail's damage are a direct function of it.

So, for the defenseless carry first: Thornmail damage = 0.3 D x (1 - 57/157) = 0.191 D Health return = LS x D x (1 - 150/250) = LS x D x 0.4 For Thornmail damage = Health return: 0.191 D = LS x D x 0.4 LS = 47.75% The defenseless carry needs 47.75% Lifesteal to negate Thornmail's damage alone

For the one with the defensive item: Thornmail damage = 0.3 D x (1 - 112/212) = 0.142 D Health return = LS x D x (1 - 150/250) = LS x D x 0.4 For Thornmail damage = Health return: 0.142 D = LS x D x 0.4 LS = 35.38% The carry without IE needs 35.38% Lifesteal to match Thornmail's damage. Even with the build with LS runes, he can only reach 34.5% Lifesteal. And said carry actually gives up his crit potential with this build.

ADDENDUM: There's a single possible way with that double Lifesteal build to outheal Thornmail: If the defensive item of the carry in question is Spirit Visage, as that item boosts all sources of healing by 20%, giving our carry effectively 41.4% Lifesteal. But that is only if the target has 150 Armor after penetration and no other form of anti-physical damage (Ninja Tabi, for example).