As King Arthur, hit an enemy with any damaging ability while their protections are reduced by Twin Cleave.
This should be quite easy. Twin Cleave is King Arthur's third ability in his standard stance (rather than his combo stance). You dash forward while spinning twice, swinging Excalibur around you in a circular motion. Once you hit an enemy god with Twin Cleave, simply follow up with any other ability that you have, as all of them deal damage, and make sure it hits the enemy you've just damaged with Twin Cleave.