Being in the air while the enemies were on the ground was a non-considerable advantage.
If the enemies didn't have any attack to endanger those in the air, then they wouldn't be able to do anything. It wasn't with a sword that someone could kill a bird when it was flying, right?
It was the same here. Since the lich was a necromancer, he had a considerable advantage over the boys.
First, since his army was already dead, they would never be tired, whereas it wasn't the case for the boys.
And second, since he was a necromancer, he could revive the dead constantly, so their numbers would always be fare superior to theirs.
And in those types of battles, having great numbers was a great asset. However, it was only the case if they could attack the enemies. Since the boys were in the air, and all of the skeletons were just warrior-type skeletons, there wasn't much chance for them to be able to attack them.
