You can sneak by the first Sentinel (MIMIR Z800) and its scout robots in the Forbidden city (and that's what the characters will say to do in game), and then engage it later by ambush in the same area. Just make sure and stay away from the scout devices and they'll eventually return to the MIMIR Z800 and you can ambush it afterwards. If its light comes near you it will put out a pulse and if you're caught in the pulse it will send out its scouts. So what you need to do is sneak up on one when it's heading away from you, ambush it, (in which case it will deploy its scout units), but now you're in combat with the advantage and you can hit all three with a single EMP grenade and it's easy pickings from there.
Here's some more tips-
- Make sure you have EMP grenades and characters with the ability to disable robotic units (although I didn't even need to use their ability since I was able to kill it within 3 turns while it was EMP'd).
- Make sure you have the Artifact Skill from Pripp's Place that lets EMP grenades last an extra turn.
- Ambush the MIMIR unit while it is far away from other enemies so you can use loud weapons. (it has a lot of armor so silent weapons won't do enough damage before the EMP wears off, although you could use another EMP grenade or your robot disabling abilities to keep the enemies stunned if you need to use silent weapons).
Note- I did this on normal difficulty so it may take different tactics on hard or very hard difficulties.
I also put together a quick video guide if you want to see how I got the achievement in video form-