Ok this one is relatively easy if you have a good group playing with you after you start extinction you will be dropped off by a chopper and given a drill to clear out hives after the 4th hive you will then be tasked to look after a chopper from scorpions this one is easy to do just load up and shoot. but the trick here is to not let the chopper go into evading mode as this then prolongs the destruction of a the barrier hive and also makes it harder for you later on in the game.
now once this barrier hive is destroyed grab the $3000 and run down the hill past the jeep on the left there is an alley way with a wooded platform use the ladder and climb up and purchase the LMG for $3000 this will give you more fire power and less reloads, then its onto the next 4 hives with this area once you have hit the later 2 which are normally near the second barrier hive always make sure the electric fence is on as well as the fire trap as this will aid you in getting to the next area and your achievement popping. after you have done hive 8 its time to ready up as this is a big onslaught of aliens, now again you must stop the chopper from going into evading mode as this will become more troublesome as the longer it takes the more likely you will encounter a Rhino. Once this barrier has gone down the achievement will pop