After figuring out the Mind-Body Sharing Technique and the Mind-Transmission Body-Sharing Technique, mastering Wood Release through Earth + Water had been a natural progression for Wu Yu. Not to mention, he also had Mysterious Kaguya's Rinnegan power assisting him from the side.
Using the Mind-Body Sharing Technique, he gradually trained the mental strength needed to focus on two tasks at once, until he cultivated [Kekkei Awareness]. Then, by simultaneously creating Earth Release- and Water Release-nature Chakra within his body and forming seals to combine them into Wood Release Chakra, it should have been that simple.
Wu Yu had indeed accomplished it step by step. But when he copied the same method and tried to use his cultivated Kekkei Awareness to arrange and combine the four Nature Transformations he currently possessed—Water, Fire, Wind, and Earth—exhausting every possible combination, he discovered that Kekkei Genkai were not nearly as simple as he had originally imagined.
Wu Yu's Water Release and Fire Release had even been refined into Yin Water, Yang Water, Yin Fire, and Yang Fire. In theory, he should now have been able to use six Chakra natures to create thirteen Kekkei Genkai through permutations and combinations.
Yet the first one he tried apart from Lava Release and Wood Release—Boil Release—unexpectedly got stuck.
His left hand refined Water Release Chakra, while his right hand refined Fire Release Chakra. The two should have smoothly merged into Boil Release Chakra, but when Wu Yu brought his hands together, the two types of Chakra seemed to repel one another and refused to blend properly.
"Are you kidding me?"
Wu Yu was speechless on the spot. "This can actually be harder than Wood Release? I got Wood Release with one clap of my hands, but Boil Release is getting stuck? Is that even right?"
Mysterious Kaguya