If I were running the thing as an encounter, an entire army of thousands of 1st-level guys wouldn't even attack, because they'd have no one to lead them.
But maybe they don't know how powerful the 10th-level fighter is. What would happen is one or two brave, foolish 1st-level guys would make tentative attacks. The 10th-level guy cuts them down in a single swipe, and the remaining army runs away.
In order for such low-level guys to want to fight the high-level guys, they need some motivation. Maybe the 10th-level guy is evil, and threatening their homes and farms. Even so, without a commander, the 1st-level guys would probably try and flee, grab their families and hide, rather than fight.
Maybe there are some 7th-level commanders (imo they'd have to be at least 7th level) threatening the 1st-level dudes with execution if they don't attack the 10th-level guy. Even so, the threat might not be enough to overcome the urge to flee. 1st-level guys do not have the kind of training required to stand and fight.
In the case of the 7th-level commanders, the encounter is not really about the 1st-level army, because to me, it's a battle of forces of will between the 10th-level fighter and the 7th-level commanders. The 1st-level guys can count as equipment, or summoned allies, effectively. Or simple reactive terrain between the real foes.