[Kill 5 enemies with one Death From Above attack]
This one can be tricky to get but can also be a piece of cake.
You actually need to kill them with the DFA attack, so you need enough speed when you hit them.
A usefull trick for it is to charge your adrenaline on a group of grunts and save it.
When you meet another group of grunts (count them and make sure there are five of them!), try to lure them to an edge or corner.
When they are all close to eachother get above them and at the top fall down and use an Adrenaline Death From Above (keep mashing Y+B).
Because of the adrenaline you will have a nice large shockwave and huge damage on small heights.
This will make sure all 5 of them die with the shockwave and ... achievement unlocked !