Stealth Kills are achieved by approaching an enemy from behind whilst cloaked and pressing the melee button when the button prompt at the bottom of the screen appears.
Getting 25 of these is very simple, cloak, get behind an enemy, stealth kill, rinse & repeat.
Tracking for this can be a bit funky - for me this stopped tracking between play sessions.
I recommend getting this done in one solid run which should only take about 20 minutes.
If your tracking has stopped working you’ll likely need to delete your local & cloud save for the game. This will allow you to start the game fresh without any tracking issues - get the 25 done (regardless of your current progress on the achievement) and it should pop.