The weapons that count for this are the Kusanagi rifles in scope mode and the COIL's secondary fire. After the penetration is upgraded on the rifles, it will shoot through moderately dense walls at an equal level of the COIL's secondary. It won't plow through thick bunker buildings, but this achievement is a breeze with an upgraded rifle, since you can take it as a loadout option. Just pop your DART-6 overlay and shoot anyone that's behind cover. Of the two loadout weapons, the sniper will be easier, as it does more damage, and with damage link, one shot's everything.