Bakudan-mon

Bakudan-mon (爆弾門) is an enemy from Mirai Ninja. These bomb gates also only appear on the seventh stage, and attack by hovering in place above the bridges while creating electrical force fields in front of Mirai Ninja (which are impossible for him to bypass without getting injured, so you should not do it if you have less than 12-15 energy points remaining), and counting down from 7 (七) to 0 (〇) in Kanji numerals; they'll then fall down onto the bridges and explode.