so for this achievement you need to go into the mission fracture jaw and its near the end where you have to land on a big hill
when you are protecting the base you need to destroy all the vehicles and mortars
the first set of vehicles are before you see the hill on a dirt road
after that you just want to look for white smoke going up into the sky because this will tell you where the mortars are
plus just take your time with the objectives ... no point rushing the objectives when you can spend your time scanning the whole hill looking for the mortars
the vehicles arent a problem because they are the objectives