It appears all "in a matchmade game" achievements can be done in Bot Bootcamp. It can also be done in ranked.
This one is about as straight forward as it gets. Kill three enemies by using the oddball in one game, i.e. punch them with it until they die. What matters is that you get the final blow and you can do this across multiple rounds within the same match. I suggest doing this against bots as you should have a much easier time - you are very slow while carrying the Oddball and you need two melee hits unless you're ninjaing them.
Addition since this seems to be a bit unclear: Bot Bootcamp works for these achievements, however Custom Games and Training Mode do not.