At that time, follow Muzzer7's advice to target four and enjoy the show! If for some reason you dont get the achievement, its possible you only targetted three.
Rinse and repeat until you have your four full focus bars (you'll know because they will all be glowing and you'll see the Super Soldier RB+A message). There is a slight load screen, then turn around, enter the estate, and do it all over again! Once the room is clear, return to the sewer entrance. Because you're using the Classic Cap costume, you will receive even more focus than usual. Swatting the bullets back will result in friendly-fire sniper defeats, which boosts your focus. You could use this location to boost your anti-sniper achievement, but chances are that you'll have that completed long before here. Use the LB deflection trick to swat sniper bullets back where they come from. Friendly fire kills bring a bonus in focus. Use whatever method you like to take out the kobolds. The other two are up above, one to the left of the entryway and one directly above it. You can expect five Kobolds and three snipers. All low level basic and ballistic guards, they dont possess too much trouble for you. Inside this area, you can expect to find seven guards. To the right is a staircase in the center of the wall that leads to a wide open area. When you enter the door for the Estate, you will move through a small hallway and up a flight of stairs before entering another hallway. It is in the northern area, second from the left in the line of 4 nearby tunnels. Enter the sewer complex, then go to the Unlocked Door for Estate. If you have already cleared the Estate level, all the better. It can be equipped from the main menu- EXTRAS - COSTUMES - Classic Cap Costume. I am not entirely certain what the requirements are for this costume, but I unlocked it very shortly after acquiring my 25,000th intel point. To refill focus quickly, equip the "Classic Cap Costume" costume as depicted in the 1940s and 1950s. Just as Muzzer7 explained, you will need a full focus bar. If you find yourself later in the game (or all the way through!) and still need this achievement, you can boost it quite quickly using a combination of unlockable costumes, sewer access, and skill.