I haven't try any other chapter yet, but the last chapter "The Fold Weapon" will guarantee the achievement (plus a few retries if you failed to wipe the army in time).

Scorch, Ion or Legion will work, since their Weapon Core can decimate a whole army in 1 sweep. I used Legion since it was easier to see the death count + ratatatatatata :D.

Here's where the fun begins. After going through a sad cutscene, melting through a bunch of soldiers as you just got the SERS kit, and go pass a dozen Titans, you'll be at the tunnel where you need to crouch, you'll need to save for the Core so don't use it near the start of that place
The last area before the tunnel, there should be a bunch of Titans here for you to earn that core

Once you're in, DO NOT attack, just keep walking in, their weapons can't damage your brand new Titan that much, until you see the red eyes from those Stalkers

At this point, activate the Core and start sweeping them down, if you see x25 kills but no achievement, restart checkpoint and do it again until you get it
That's all! (No achievement pop-up in the screen cuz I already got it)