Even though your last confrontation with the Witness ended in retreat, you were still able to land a devastating blow—proof that your newfound powers are strong enough to defeat it.
The Witness was not the only one wounded by the encounter. Ghost's condition has rapidly deteriorated, and it's clear to you that he's only holding himself together so the two of you can finish this.
And you'll need the Traveler's full power behind you to do so.
Work with allied reinforcements to weaken and sever the Witness's hold on the Traveler. Only then will you be able to use your mastery over both Light and Darkness to unmake humanity's most ancient enemy.