Was her Dark Seal fully stacked? What level was she? 9? What was her runes?
Assuming Domination first, Sorc second, she probably had 163 AP (assuming half stacked eyeballs and full stacked darkseal) and assuming W max first (since i can't find the game).
Electrocute: 179
Luden: 116
Q: 120
W: 343
E hit: 89
E root: 174
R (assuming E): 135 and then 270 on root
This adds up to 1426 damage with my assumptions. I might be somewhere wrong. However, I'm going to assume you're level 9 as well which means your base HP is 1166 + 60 HP + 70 or 350 (if you maxed E first) so either your max hp was like 1296 or 1576.
I don't know if she bought boots, but assuming she didn't she has 8 penetration from Sudden Impact so you essentially had 25 MR which means you take 80% damage.
so roughly 1141 damage + however many autos she threw at you (I am guessing 3 because those roots take a bit so another 236 damage).
tldr; This all adds up to LeBlanc is kind of BS for how safe she is (they reworked her for a reason but forgot i guess lol) and Death Recap is garbage at telling you actual numbers.