This counts kills of enemy worms across all game types. There is no in-game counter or progress indicator, so you should get all other kill related achievements before trying to grind this one out, including Worminator. At that point, the fastest way to get kills is in a local Versus deathmatch set to three wins against three clones. Choose the Single Island or Two Islands template using the achievement game settings from the road map. Pile all of the worms in one spot with access to the open sky. To speed up placement, you can include your worms in the pile without consequence. Pass until it is your turn and hit the pile of worms with the Concrete Donkey. You can avoid the extended death scene of each worm blowing itself up if you position the pile of worms on the edge of the island with a slope angled down toward the edge of the map. This way the impact of the Concrete Donkey will tend to blow them into the water, with no death scene. Rinse and repeat.