So, this applies for every achievement that is not menu related in this game, all you need to do to 100% endless furry killer 2020 is to get to wave 50.
You must play on the hardest difficulty as there's no reason to play on easy, it's not more difficult per se, the furries just have more health; you'll need to upgrade your damage to have 200 per shot so you can one-shot every enemy.
Now, every wave has a one-minute cooldown before you can call on the next wave, upon reaching wave 50, you'll start encountering a boss enemy, the Senator of Michigan (I think, I ain't wasting another hour) who upon being killed will give roughly 200,000 points. Kill a couple of senators until you reach one million points. Then you can happily let yourself be mauled to death by the angry furries to get the death achievement.