-Killed all enemies in the beach houses without causing an alarm ("Leave No Man Behind")
At the very start of Act 1, mission 3 there is a house on the beach and some guards patroling there. You need to take them all out without being noticed. The objective gets completed when there is still one guard left (the one who is looking at the sea) but you still have to kill him too to get the achievement to pop. You can restart if you screw up.
I think the easiest way is to go straight to this little legde and snipe the guard on the pier. After that continue to the left and take out the guy in the small building there. Continue your way down the road and kill the guy upstairs. Then wait for the patroling guard to get close to you to take him out and then just shoot the one on the door. After that there is only that one poor guy who misses the sea and haven't noticed that all his buddies are dead. Take him out for the achievement.