Devil May Cry Wiki

In this Secret Mission, Nero must slay 8 Scarecrows within a minute. The fastest method to kill each of them is to perform:

Button ps4 triangle, Button ps4 triangle, High Roller, Button ps4 triangle (mid-air), Buster (mid-air)

This combo is stylish enough that it kills a Scarecrow each time. After defeating each Scarecrow, use Snatch to grab the next one.

Using just regular busters and snatches without using Red Queen can also work, however this takes slightly more time than using combos. Using Exceed can also be useful, and having MAX-act can be a quick way to fill it up and do some major damage. Once you have killed the Scarecrows, you can smash some garbage cans and benches, and collect your blue orb fragment.