Luke couldn't exactly beat Abeloth 1 v 1 with Oneness
"But the light side rushed in, flowing in from all sides, pouring through him like fire. A golden glow began to rise from his skin— cells literally bursting with the power of the Force—and Luke felt them both start upward again. Abeloth countered, hissing in anger, and they hovered a hand span above the floor. A tremendous crack echoed down from the vaulted ceiling. They dropped again, hitting so hard that Luke’s breath left in a groan. Abeloth slammed down atop him, her single leg still wrapped around his, the stump of her arm driving harder into his throat. Something crunched in his larynx. His breath came in shallow, wheezy gasps, and the crushing hand of panic began to clench at his heart."-FOTJ: Vortex
Well, yes, he can't beat her with oneness. Oneness is not a destructive power, it can greatly enhance physical and Force abilities but it's not going to kill opponent on it's own. Point is Luke killed Abeloth with lightsaber more than ones. That's why it matters, if Luke has lightsaber during fight with Daughter or not.