1. Head for the Hanzo Academy storyline

2. Go to its Story mode (Shinobi Girls' Code -Hanzō Battle Records-)

3. With Hibari (on any level), choose the first mission on the first chapter on any difficulty (Basic Training)

4. As soon as you're able, you can just immediately jump and then spam the Weak Attack which should just result in all the oncoming enemies coming to you ready to take an eternal beating. Just keep doing that until you reach 7,777 hits which should immediately unlock the achievement.

If you have to you can group 'em somewhere or something.

Another idea is reaching Chapter 4 Mission 4 during Hanzō's Story mode where you're supposed to survive "3 minutes" against an endless amount of spawning enemies before the mission's boss arrives. Because the timer's been fixed it's harder to use Hibari's endless buttslam here to reach high combo numbers but after you beat that mission regularly you can go through that mission with Yagyū afterwards and use her Frantic mode which can dish out high combo numbers pretty fast.