Level 99 YRP, Speed Bracers and Sprint Shoes equipped, Thief DSP, Howling Wind Garment Grid equipped if you have it. Fight an oversouled Chac. Get into a rhythm of having the girls attack after each other and hold down X. Still technically possible at lower levels or without all this stuff, but your inputs will have to be timed tighter. There is less margin for error.
You can also do this with a max Agility, Quick Attack spamming fiend on Almighty Shinra, but unlocking him is time-consuming and unrelated to any other achievements.