Does earthen armour actually work?

I was doing ambuscade earlier and asuran claws did 2k, 1 shotting August. So next time I put earthen armour up to prevent the 1 shot.

If I understand the move right, if an attack does 75% of your max hp in damage (which would be about 1.5k for August) it reduces the damage by 45%.

But with earthen armour on the asuran claws still did 1.8k, killing August. So does the move just not work or have I misunderstood it's effect?

Asuran Claws: Delivers a sixfold attack. Accuracy varies with TP.

Earthen Armor: Reduces any single action that would have taken over 75% of your maximum HP by 45%.
Multi-hit monster attacks do not count as a single action and will only proc if the individual hits exceed 75% of your character's HP, so it is still possible to be one-shotted by a multi-hit attack.

So if I had to guess, and assuming the mob move follows the BLU spell, it wouldn't work on Asuran Claws.