Escape Dead Island

Just as the achievement description says, you'll need to kill 100 zombies to get this achievement. Unlike in previous Dead Island games, there is no achievement tracker in the pause menu, so you'll need to mentally keep track of how many zombies you've killed if you want to have some way of tracking your progress. That said, I don't recommend doing that unless you're really picky about knowing your achievement progress, because killing 100 zombies is pretty easy, and will unlock relatively early in the game.

Zombies can be killed via stealth kill, your melee weapon, your pistol, your shotgun (although you should already have this achievement by the time you acquire the shotgun) or executions (if you've managed to knock a zombie to the ground). Keep in mind that if you die, you'll lose all progress towards any achievements, as well as any collectibles or upgrades you've gotten, since the last time the game autosaved (which also prevents you from reloading saves to farm kills).

As you're working towards this, you can try to go for the other weapon-specific achievements, listed here for your convenience: