I Think Julius is the strongest character we have seen. Probably only the Demon is stronger. I'm also pretty sure that the first emperor would have won against Julius because of the match up, but i really think he is OP. He overwhelmed Patry without even trying to wound William's body, he demonstrated that time magic it's unbelivable, that he has an incredible control of mana zone and also that he can read the future(somehow). I really do not know how someone could kill him, yeah, we know patry did it, but with a trick. I'm pretty sure that Only the Demon and the first emperor have the power to overcome Julius, but i'am pretty sure that Julius is even stronger than the first emperor and that he would lose only for the match-up, like Yami could stand against patry only for the same reason(and we should remember that if there wasn't Gauche both Yami and Asta would have been killed).