Who was The Power that struggled mentally with Galadriel at Dol Guldur?

[Angry Hill Troll]

I seriously doubt that the Witch King of Angmar was the one that Galadriel was wrestling in thought with. The only one that could prove a challenge for her is Sauron, not one of his minions.

Tolkien says so in the UT, referring to Denethor's and Saruman's use of the Palantiri:

It was not [Sauron's] way to commit such instruments to the use of subordinates; nor had he any servant whose mental powers were superior to Saruman's or even Denethor's.