Uncapped multiplier:

<Damage Formula>
// This is the damage formula used for the skill.
value = a.atk * 2;
// Get the HP difference between the target and attacker.
var difference = b.hp - a.hp;
// Check if the HP difference is greater than 0.
if (difference > 0) {
  // Get the value per 50 HP difference and round down.
  difference = Math.floor(difference / 50);
  // Add 10% bonus rate per difference.
  var rate = 1.00 + 0.10 * difference;
  // Apply the bonus rate to the calculated base damage.
  value *= rate;
}
</Damage Formula>

Capped multiplier of 150%.

<Damage Formula>
// This is the damage formula used for the skill.
value = a.atk * 2;
// Get the HP difference between the target and attacker.
var difference = b.hp - a.hp;
// Check if the HP difference is greater than 0.
if (difference > 0) {
  // Get the value per 50 HP difference and round down.
  difference = Math.floor(difference / 50);
  // Add 10% bonus rate per difference.
  var rate = 1.00 + 0.10 * difference;
  // Set a maximum bonus rate of 150%.
  rate = Math.min(1.50, rate);
  // Apply the bonus rate to the calculated base damage.
  value *= rate;
}
</Damage Formula>

Happy Giant Slaying!