You can get get this in the FIRST chapter. When you get outside you have to make your way to the car, DO NOT DO THIS run up and get to the left of Captain Robert and kneel behind the concrete bariccade they can not hit you here (think kneel is press down on right control stick). Once here you can head shot the guys behind their concrete bariccade. Put your sight over the area where their head will be when they stand up and shoot them. The pistol can drop a guy in 2 bullets so just quick tap it, and the laser weapon can drop them in one. Try to get at least 13 to 16 kills here. You will run out of ammo. At this point run to the car and trigger the cut scene. When you get control again run up and grab 2 ammo packs then run back and kill the 3 guys above you with head shots. From here just use the laser weapon. Now move forward and kill the 4 guys in the underpass. You can try and head shot the grenade suicide guy as well. Continue forward and to the next area where the guy kills the woman kill him and guy above you to the left. Move forward and kill the guy before the freeway area. Collect the artifact and climb the ladder run around to the right and collect the gear. When you walk back over the bridge over the freeway below you 4 guys will spawn on the freeway who you can kill from up here. If you did well in the first area the trophy could pop here. If not continue forward in the game. The next time soldiers show up is when you arrange the train. After you arrange the train DO NOT get on board yet, wait and 4 more guys will spawn at the 2 doors on either side of the train. Kill them and board the train. If you still do not have it head shot the guys on the train and hopefully you will get it before the end. There are 23 soldiers AFTER the grenade suicide guy blows up the car so you need to have killed at least 7 at the very start to make this work.