This is done in Chapter 9 "Hidden Pasts" - Level 1.
During the level you will arrive at a Hellghast Missile Base. On the left side of the big gate you can see that the wall is damaged. Enter the base at that point. There will be two Elites to your left so watch out. After you took care of them walk a bit further and you will find a Siren (it's a mounted Grenade Launcher). Use it by pressing
and fire away on three APCs and a bunch of Hellghast on foot. You will not be able to get 25 kills in one go meaning you have to hit
and Retry from last checkpoint
For those of you who prefer visual...
Video credits to sunnygrg