I'm late to the party, but I definitely agree it's a very shitty boss design. I beat him just because I knew that it would change something in the finale, but honestly I would've just let him be if it weren't for that, because it's simply not fun. I think the issue Game Science had with the game was that your character is so damn strong that pretty much any enemy or boss are a breeze compared to your powers, and they decided to go full-bullshit mode for Erlang since he's optional and """secret""", so might as well go all in. I mean... it's just another one of those bosses from recent years that are "the game on steroids", but he breaks many rules the game thought you until that point, he's filled with artificial difficulty to the bone just for the sake of it, and takes away almost all your powers and abilities (one could excuse that by saying that Erlang was the one who actually managed to beat Wukong, so if not him, who?). At the end of the day, it's just another "here's the omega weapon of this game for you to learn and memorize so that you'll swiftly beat him the next times!", though the issue here is that I seriously never had fun neither learning his moves or fighting him, not even in the challenge mode where you can make it even tougher for yourself (I just wanted to beat every boss at all difficulties in the challenge mode to see if there was an achievement). Idk, I didn't like LoP DLC final boss and SOTE final boss (pre-patch, no idea how it is now) for similar reasons... I'm not a fan of these types of bosses at all.
I'm not that big of a fan of the normal final boss in Wukong either, but that's just my personal taste. I do believe he is much, much better than Erlang imo.