To receive a x3 multiplier, you must kill an enemy within 5 seconds of the contract starting. A good method is to take note of the color of the boss's shirt on the contract detail screen before loading-in. Quickly scan the scene for him and zoom in and fire as quick as possible. You can also just zoom quickly onto any enemy and kill them, but you risk failing the contract if the target escapes. This achievement is glitchy, again for the good, as it unlocks only after getting just five x3 multipliers. You can check your progress in the stats under "X3 Multipliers Gained."