Oh, this question is super easy asdkfhdsjskl; 🤣

<Ashterot the Grimmsnarl> and <Nyrgal the Delta!Ditto> are both the most treacherous of Elio's entire roster; both of them are sadistic assholes at their core and they're not afraid to backstab their master at a moment's notice, despite being fully aware of the consequences of doing so. Though, when Elio relishes the thought of physically fighting the two should that line is ever crossed, he commented that at least it would be a fun challenge - given their unique qualities.

Bonus: <Molesurya the Volcarona> would also fall into this category, but since his "Berserk Button" is Elio - more specifically when his brother's life is in severe danger, which already happened once before and the trigger for his fiery evolution, that fight clearly wouldn't happen.