Loki's strength level is indeed far below Borak's. If Borak hadn't used the soul-reading spell to forcibly probe William's memories, Loki wouldn't have been able to escape the ice prison.
But Borak was asking for trouble. Of all the things he could have done, he chose to use his psychic powers to explore William's memories, which was a very bold move.
Not to mention Supreme Deity level, even high-ranking god-kings would not dare to probe William's memories.
The fact that he didn't drop dead on the spot was already lucky.
William was merely instinctively rejecting the intrusion into his memories.
Even so, Borak was left delirious, allowing Loki to do as he pleased. The Little Broken Stick, being an ancient Dark Artifact, was no ordinary object. Its demonic aura voraciously eroded Borak's flesh.