Either you want Illaoi to do enough damage to instantly kill just about anyone, or you are confused about what is meant by "Tentacle hits beyond the first deal 50% damage."
What this means is that if 2 tentacles hit the same target at the same time, one of them will do half damage.
If 3 hit at once, two of them will do half the damage of the first one each.
In this way, 3 tentacles will do 2x the damage of 1 tentacle, and 5 tentacles will do 3x the damage of 1 tentacle.
If one tentacle hits someone multiple times, each hit will do as much damage as the first one.
The ability works this way so that when Illaoi ults it doesn't result in an instant ace. If you got a full team ult and each tentacle did 100% damage, then the enemy team would at the very least be massively chunked without having any time to react.