A boy with the power to create the strongest weapon meets a girl who must take his sword in her hands to save the world.

From vyc at MangaHelpers:

In the Ben-to series:

V.1 - Ben-to Zero: Road to Witch
Bargain hunters seek the best deals and compete against each other to obtain the product on sale. Yarizui Sen, who just moved into town and started living on her own, is introduced into the cutthroat world of bento bargain sales.

Near closing time, supermarkets put unsold bentos, prepackaged meals, on sale and hungry competitors who call themselves wolves fight against each other to see who gets the right to purchase the delicious and cheap meal. Sen learns under the most feared competitor of the town and trains to use her hunger as a source of obtaining the half off bentos.

V.2 - Ben-to Another: Ripper's Night

V.3 - Ben-to: À la carte