Thank you for your question. Annihilating in God's Essence is impossible to conceive of as the human, who is essentially a limited being would have to become unlimited. This is a contradiction as something cannot be limited and unlimited at the same time. Rather, what is meant by the term annihilation is the annihilation of the ego such that all that a person witnesses within their being are the signs of God. Those signs, which manifest divine perfections, seem unlimited to us as their limitations are beyond our perception. At this stage, a person only sees God through the mirror of their soul. This view is compatible with Shii scriptural sources such as this part of the munajat of Shaban:
"My Lord, (please) grant me absolute devotion to You and illuminate the vision of our hearts by the light of their gaze looking towards You. Until the vision of our hearts pierces the curtains of light to reach the core of supremacy and that our souls become suspended by the majesty of Your Holiness."
May you always be successful