MAG
44 Achievements
449-549h
PS3
Silent but Deadly
Awarded when a player kills 25 enemies with Poison Gas over a career.
3.5%
How to unlock the Silent but Deadly achievement in MAG - Definitive Guide
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This one should come over time. Use the gas grenades (unlockable) or the gas mortar attack on crowded areas (when you're a Platoon Leader in defense). You only need to kill 25 enemies, so it will come quickly.