Apart from the Guru, do you think that a mediator is necessary in order to enable us to attain Self-realization?

Yes, yes. That is the Tutelary Deity or Ishta Devata. The mind cannot all at once rise
beyond itself. The ego will seldom cut its own throat. Your consciousness will find it a hard
task to realize the Cosmic Consciousness. Therefore the Name and the Form of a Deity are
chosen to meditate upon. In due time, this Deity will manifest Itself before you and do the work
of annihilating the ego and the mind, enabling you to realize Cosmic Consciousness.

From "May I answer that" by Swami Sivananda (1987-1963). More Informations on Swami Sivananda: on the Website of Divine Life Society, Photographs of Swami Sivananda, German Pages on Swami Sivananda