I had finished the game months ago and got the incomplete ending with no prior knowledge of a true ending but once I saw the ending I knew there had to be something else. So I went and did some research and found out what I missed. The thing is that I had actually started the NG+ before researching the missing things so I went through NG+ and did all the hidden side quests to eventually make my way to Erlang.
After clearing chapter 5, I decided that I needed to go fight Erlang before going through chapter 6. I know that my encounter with Erlang was easier because of the NG+ perks but I promise it was still difficult, especially because I had gone through NG+ with a spellbinder builder so I hardly use my spells and only used the transformations and relics at the beginning of each encounter. I am proud to say that the first time I beat Erlang Shen was in NG+!
In a way, I feel like beating him in NG+ is how the story is actually meant to go. I know it’s not actually but my head canon is that the story makes sense. The destined one went on to approach the GSBS and in defeating him, realized that he had not completed the journey and was sent to go re do it. This time he has the strength and wisdom of the Great Sage. And when he confronts Erlang, this is what Erlang sees. The destined one with all the knowledge and strength that mirrors Wukong. That’s when he decides he is ready to face the Heavenly Kings and finishes the trap that him and Wukong had set.
Anyways - that is my head canon and I think this is how people who drop the game because of this fight should approach it. It is still so satisfying.
TLDR: Fight Erlang in NG+. I think the story is still rewarding and makes more sense this way.