In Strange Brigade, in addition to the many collectibles, there are also some other trophies / achievements that you can unlock. If you want to play the Platinum Trophy or 100%, you must also unlock success in Strange Brigade.
These Are My Fighting Trousers
Melee kill 5 enemies in 30 seconds
An overview of all Strange Brigade PS4 trophies, you can find here
Melee 5 enemies in 30 sec
The fastest and easiest way is at the beginning of the mission "Forsaken City". There are 5 enemies already on the ground and you just have to press the button.