True...I did more math though for a super unlikely situation.
So if level 18 full AP Lulu (six deathcaps, full scaling AP runes, all AP masteries, Baron buff, Five Dragon stacks, etc etc...) ults Ekko, that's 600 base HP plus 656 from all of her AP for a bonus 1256 HP on our Ekko from above. Add 211 flat HP from runes/masteries, 250 from Elixir of Ruin, and 12% from runes/masteries, and our Ekko has roughly 6784 HP. Ekko then takes 6783 damage within a time span of four seconds before using his ult. 50% of that damage taken is 3391.5, plus 10% from the Second Wind mastery for a whopping total of just about 3731 HP healed instantly. Still unsure if that's more or less than the Zilean ult.
Also it's very possible I did the math in the wrong way.