Easier start on Atmospheric Conversion Center (interior)
When starting level 14, attack the enemy which is in front of the pillar that you started out at. He will attack you as well. When he does this, run behind the pillar. This will "bring" his other patrolling friends towards you. This will be a bit easier than running into the open, where a sniper is waiting for you. When all three of the enemies appear, duck and continuously trip them. This strategy works if you disarm the man carrying the gun. After defeating them, look to the left of where you started. There will be an invisibility item. Pick it up and run towards the exit where two guards are located. If you try to kill them, you will probably get shot down by the sniper.