Just did some math...
At rank one of his ult, you have to have 2884.6 missing health.
At rank two of his ult, you have to have 1952 missing health.
At rank three of his ult, you have to have 1187.5 missing health.
Between you and me, I think it's fair to ignore the first two ranks because if you have that much missing health in the early/mid-game, you're probably going to be killed by anything from a Sona ult to a Teemo shroom.
Rank three, admittedly is a bit ridiculous. Assuming the villain is a carry, which typically have base health at level 18 ranging from 1900-2500, 1187.5 missing health is dangerous if you're within a bruisers/juggernaut's danger zone regardless if they have an execute or not.
But yeah, thought I'd tackle that. You have to be low for his ult to do massive damages, but it's an execute for a reason.