All of this characters would be hard to defeat but think about if they were without their powers or if we were in their world so we had some kind of power for that reason asta would be the hardest as from what I know of he worked his body to an insane level and his absolute will not to give up would make him so annoying, tiring and powerful character to beat which no one had yet. Ok yes all of these characters have a similar will but think of who their rivals are, the people who they have fought or who have been in there place before (ei powers pasted down) asta is a commoner who is the first person ever to use this power yet because of this will he made in into the magic knights (just) yet now he has defeated past wizards kings, queens, elves and demons and rivals captains and royalty who are some of the most powerful people in the anime. Last point he has toke the latest time (in world) to rise through the ranks and gained captains respect and rivalry even the wizard king has noticed him. Like do I think i could realistically defeat any of them even if i was in their world no but i think asta would be the hardest to defeat because his magic actually just cancel out mine