He already knows a spell like that, he is looking for a buff spell rather than a one time magical attack
Thinking about an element, earth should be the best choice, it's the only one that is similar to a physical object. Using water, fire or wind to extend his sword will require a lot of mana, because without using mana to sustain these elements, they naturally dissipate.
The idea is interesting, the execution is amazing and the quality is high, not to mention the insane chapter release speed. In conclusion: great story, great speed, great author.