Lex had trained and strengthened all his abilities, so how could he leave Mindmeld behind? But compared to his other methods, which could be forced, Mindmeld required an element of subtlety - at least for those in a similar realm as himself.
This could change if Lex drastically increased his spirit and mental power, but it was something he had never focused on before, so he was not at an overwhelming advantage for now.
That is why, the moment Lex slapped the wyvern, he had actually already put him under the influence of the Mindmeld. But, since he was concerned that the wyvern might be able to detect signs of infiltration in his mind, within the illusion as well he made himself try to search the wyvern's brain, while allowing the wyvern to secretly resist.
This way, even if the wyvern detected the intrusion, it would feel like it was successfully hiding from Lex.
