You can get this in the 12th mission, Old Comrades. Luckily, there are many different enemies carrying a wide variety of weapons. Progress through the mission, checking each time you kill an enemy to see what weapon they drop. If it's a weapon you're not using in this mission, pick it up and get a kill with it. I found at least a dozen different weapons in this mission. .50GS, X16, 1911, MP7, AK47, M19, Uzi, RPG7, FAL, P90, Model 680. Get 1 kill with 8 different weapons (grenades not counted) and escape in a van to unlock the achievement.