The Dishwasher: Vampire Smile

You aren't given the option to chainsaw duel right away until you hit those Blue-eyed pumpkin monsters a little bit into the game. The reason for this is because, they're the ones with chainsaws!

Yuki, having a lovely chainsaw arm (well, more accurately a machine arm but that's not the point) can chainsaw duel these enemies. So yes, you MUST be using Yuki to obtain this achievement. While fighting them, commonly after a few hits, you'll see a command pop up to press Y. If you manage to do it, it'll go into a chainsaw duel which will have you spamming the Y button until you win.

They are pretty quick duels and there's more than enough pumpkin guys between when they're first introduced and when the game ends to get the achievement.

Using Yuki, play the level The Marina. There will be several rooms with the chainsaw-toting pumpkin minibosses. Attack them until you see the Y action command light up, then press Y to engage them in a chainsaw duel. Just mash away at Y and you'll win.

There are four opportunities in The Marina. Takes less than ten minutes to get this achievement.