Ponkotsu ga Tensei Shitara Zongai Saikyou

Shota Kanda is your average 16-year-old first-year in high school. On his way to school with his hot but useless childhood friend Sanae Kondo and his big-chested but shy friend Kana Funami, Sanae's strength gets them killed...literally. They wake up in another world with powers that are not to be believed—Shota's world-class defense against getting slapped becomes world-class defense against demons, Kana's now-magic chest heals all their wounds, and Sanae? Her strength has turned to something truly terrifying. Will the three be able to live in this strange new world, and someday even get back to their own?
(Source: Kodansha USA)
Note: Chapter count includes 6 extras.