In higher levels try adding alot more C4 into the mix. Once alot of enemies has clustered on or near the areas you've planted your C4 on, blow it up and you should get a nice sum of cash. Make sure you stay close to the wall and keep an eye on you radar. That done head into small kitchen counter area and go prone.
One near the door that leads to the fenced lawn type area and one near the make-shift ramp that leads down into the floor below. Buy C4 (costs 1500 per five) and head to the building in the middle of the map( shouldn't be too hard considering tha it has a collapsed floor) and plant C4 in the two sections of the room. Once achieving the required level and having enough for a full set. On the map Resistance, there is an easy way for a player to earn a rampage bonus.