Research centre combat master
Very rare2.6%ObjectiveOptionalAnywhere
Achieve gold medal for this challenge map
Challenge maps are unlocked as you progress through the story. You need at least "I am Death Incarnate" difficulty to be eligible for Gold score for the achievement. Concentrate on headshots (+200/kill), and just try to get combos of 2-3 enemies. Headshots are the most important. Then the Golds won't be hard :)