A head commander's defense was just as good as their attacks, and their stamina was great too. Even without combat qi, fighting for half a day with only knight skills should be ok.
This was not the case for Wizards. All their power came from their magic power. When all their mana was used up, any physical impact could greatly affect them. Wizards could use defensive spells to increase their own defense, as well as attack using magic elements.
A wizard depended on their mana, and normally their mana would be used up quickly. Therefore, every wizard needed a head commander to go along with them to Mount Budapest.
No one had the skill of a head commander and be a wizard like Abel, not to mention those powerful summons of his. Therefore, only Abel could fight alone like this.