Spoilers
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Can Only Be Completed Once You Have Entered The Sahara Laboratory.
In The Labs, There Are A Number Of Hallways With Sprinkler Systems In The Ceiling.
Lighting An Enemy On Fire, Shooting A Flare Down The Hallway Or (Occasionally) Lighting A Torch Or Weapon On Fire - Sets Off The Sprinklers, And Nets You The Achievement.
Good Luck!