This stage includes a range of the highest spiritual achievements — from the first Samadhis — up to Mergence with the Primordial Consciousness and with the Absolute.
The consciousness of the spiritual seeker prepared at the previous stage becomes capable of getting in contact with the Consciousness of God in the highest eons. These first contacts give one a vivid novelty of bliss, which is what the term Samadhi denotes [6,11].
In contrast to Samadhi, Nirvana is a stable Mergence with the Consciousness of God in which the feeling of the localized “I” disappears. The term Nirvana means “complete burning away”, i.e. losing the individuality through Mergence with God in the aspects of the Holy Spirit or the Creator. And it really happens.
In the Bhagavad Gita, Krishna speaks about Samadhi and about two principal stages of Nirvana: Nirvana in Brahman (the Holy Spirit) and Nirvana in Ishvara (the Creator).
But in India, the term Nirvana became widely used by Buddhists at some point in time, and later on, this term along with Buddhism, was “forced out” from India by Hindus. Instead of using the term Nirvana, Hindu schools started to expand the meaning of the term Samadhi by adding to it various prefixes. Various schools used these composite words, and because of this the term Samadhi became “diffused” and lost its unambiguity. This is why it makes sense to get back to the accurate terminology that God introduced into spiritual culture through Krishna.
So, in order to get from Samadhi (Bliss of Contact) to Nirvana (Mergence) one has to have a large and strong consciousness, developed by preceding trainings. In addition to this, it has to be firmly established in Divine subtlety.
If these conditions are fulfilled, then all one needs to do is just to find an entrance into the required eon, to enter it, and to dissolve oneself in its Consciousness using the method of total reciprocity, which one has to master in advance.
This task requires not only meditative skills but ethical preparation as well: destroying the lower “I” in every possible way and replacing it with the collective we first, and then with the universal “I”, i.e. with Paramatman.
This is the only way man can connect to the unlimited Divine Power.
“… We have an inexhaustible reservoir of psychic energy!”  (Hierarchy: 394), says God.
But “if one were to expound the conditions and the aims of Yoga, the number of applicants would not be great. Terrifying for them would be the renunciation of selfhood…” 
In connection to the above said, I want to cite the Carlos Castaneda’s book The Power of Silence: “… War, for a (spiritual) warrior, is the total struggle against that individual “I” that has deprived man of power.” (see ).
… One explores the highest eons of the Absolute one after another. Before starting exploring the next eon, one has to accumulate the power of the consciousness for a long time, sometimes for years, in order to be able to enter it and remain in it. The only exception is people who approached these stages in their previous incarnations and maintained the necessary amount of personal power and the level of the subtlety of the consciousness.