This is a simple achievement that will just take a bit of time. I would generally just check the game once or twice a day after work or leave it run next to me while doing something else. I as able to get this in about a week of slow progression.
If you just want to focus on grinding out these kills (which isn't necessary if you are going to go for other achievements) you can turn off auto progression and stay at an early level where you one shot all enemies and set it so your xbox does not turn off. Another option if you are actively playing is keep resetting your free play runs and putting all your money into click damage until you can't progress, then repeat. This will help you work towards other achievements as well.