Kill a Brute using only melee damage while under the influence of Saints Flow.

"After The Military ambush Jimmy tells you to go to the nearest Planet Saints to look for Johnny. Once you get there the Morning Stars will start to attack you..."
After that about 5 Brutes come with them (the Morning Stars), then all you have to do is go up to one and melee them with the R1 Button.

The trophy will be awarded once you have kill one with just melee.