Executioner
Killed 10 enemies in a single activation of Showdown.

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Nisha the Lawbringer's Action Skill is called Showdown and to get this trophy, you must get 10 kills in one activation of the Showdown. Be sure to upgrade the Saddle Pp Skill to increase gun damage with each kill and the Ruthless skill to extend the duration to 0.5 seconds per a kill. You can also find class mods to increase this even further. The Side Mission Guardian hunter is a good place, do not capture the guardians and keep killing them as soon as they spawn. If Nisha is not your main character then try getting this during the Zapped 2.0 side mission which comes very early in the game. If you keep killing Torks then a Tork Queen/Empress will spawn and produce a bunch of baby Torks which will come right at you.

Here's a tip by Keyser Soze:

I got Executioner very early in my Nisha play through during the Marooned mission. I got the first batch of enemies in Regolith Range to follow me behind the building and towards the next big wave of enemies (heading south towards the breaker for the jump pad). Once I had the second mob swarming me I went into Showdown and easily got more than ten on my first try.