Description of the passive: Will decrease damage from a single enemy hit so that incoming damage will not exceed 50% of MAX HP. Can occur once per enemy attack. 

WTR: attack Rotos with a champion that can deal over 50% of his HP as damage and does 2 or more hits.

Expected: the damage of the first hit of the attack is mitigated to 50% of Rotos' max HP, 2nd and following hits do full damage.

Actual: the damage of each hit is mitigated to 50% of Rotos' max HP.

Example: if Rotos has 20k HP and 40k shield, and is attacked by Gala who does 3 attacks, each of her hits will take 10k of the shield, leaving Rotos with full 20k HP and 10k shield.

In the screenshots, Rotos with 38 818 HP was attacked by Gala. Each of her hits on him did exactly half of his HP, 19 409, because the damage of each hit was mitigated.