This is quite an easy mission to finish, just read this guide and try your luck. There is a high chance that you’ll accomplish it swiftly.
The goal is to kill one-hundred enemies while in Marauder Scout Walker, chapter: Heavy Metal.

So here are some reminders you must ponder on:

1. You first gain access to the Marauder Scout Walker in the chapter Heavy Metal. In this chapter, there are about ninety enemies and another ninety or so two chapters later in Must Go Faster, so it is easily obtainable through natural progression.

At the start you will encounter a couple of enemies right away. Fire chain lightning beams at the enemies then enter the area

Get to the Marauders

2. Once your kill count reaches one-hundred, just a few seconds later, the trophy will be awarded to you.

You can see a short video of this walk through HERE