Nezha was already quite powerful when he was still at the Celestial Court back in the Divine Dominion Continent. He was no weaker than Erlang Shen.
After he was killed by Sun Wukong, he was then reincarnated in Dragonquarry.
He had been growing stronger when he was stuck in the Hundred-Ghost Forest. He worked extremely hard so that his soul could reach the level of a Divine Saint.
Other than his own hard work, the other reason why he could become so strong was because of Jiang Feng.
If it was not because Jiang Feng had helped him create such a perfect body, he would not have become so powerful.
His flesh alone was already at the Pre-Creation Spiritual Artifact level. And then, he obtained the Shifter Emperor's bloodline.
No one could compare to him in Divine Dominion in terms of his body.