For this achievement, you are required to kill 200 enemies with the Mega Buster. This does not count the ranged attack that is performed with the right trigger.
To get kills that count towards this achievement, you must use the skill move that is unlocked through building up your combo meter. Once your combo meter is full, you can use
+
to unleash a charged blast at a large group of zombies. Once you have killed 200 enemies, you'll have your achievement.
This will more than likely come with natural progression if you like to use the X costume.
Here is a video guide for those who may find it to be useful:
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