Below is a guide to completing the Cluster Kill achievement on Gears Of War.

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Cluster Kill - Achievement
To get the achievement killing three enemies at once ten times, go to the stage Act 3 - Evolution and locate the area of the Imulsion plant where the guy gets killed when he falls through the rotten wood floor.

Take along at least one grenade and simply follow the dead guy down a hole (or any hole, if you step in the wrong spot). Run around the cellar until you think you have enough enemies following (meaning at least three running after you), tag one with a grenade melee attack.

If you die, that's perfect, since you restart right at the checkpoint of that room and do it again. Achievement progress are independent of game progress. If not, reload the checkpoint and do it again.