Escape Dead Island

As the achievement description says, you'll need to stealth kill 25 zombies to get this achievement. Unlike in previous Dead Island games, there is no handy achievement tracker, so you won't be able to see how far away you are from getting this achievement unless you mentally keep track of it.

To stealth kill a zombie, crouch by clicking , hide behind something (usually a crate or a fallen table). The zombie's attention meter (the exclamation point above its head) should not be red - wait for it to quiet down to an orange, green, or non-existent. Next, while continuing to crouch, creep up behind the zombie. You'll then receive a button prompt to push to stealth kill. Don't worry if the exclamation point reappears or even turns red as you're approaching - you just don't want it to be red right off the bat. So long as you stay crouching, and approach the zombie from behind, you'll be able to stealth kill it.

It's also worth mentioning that if you're in the middle of a stealth kill animation and another zombie spots you or starts to hit you, you CAN take damage, and you can die (spitters are particularly bad for this, since it's really easy for them to hit you from across the map). If you die, any stealth kills you've gotten since the last time the game saved will be lost (same goes for any collectibles or upgrades you may have gotten since your last save). So if it feels like it's taking a while for you to get to 25 kills, don't worry, you'll get there eventually, it just might take longer than 25 actual kills if you're dying frequently.

Hope this helps