The Bookwalker: Thief of Tales

The cops are in Book 5 - Timeless Mansion and they're tied to the story, so you can't miss neither of them.
There are no fights in this book before them and only a few places where you can spend ink but it's better to be prepared and always have your ink level at maximum. And of course, you should've been upgrading your skills after every book up to this point.
During the fights I almost exclusively used a mix of Stun and Drain Ink (after the upgrades they both dealt some damage too), basically stun when they want to attack and Drain when you're low on Ink (or better, use a flask of ink, it's not really needed in this book for anything else). They can hit you with non-damaging spells for the achievement to unlock (block skill or drain) so plan your moves accordingly.
And although I've managed to defeat them the first time, I'm guessing you can simply quit the game and reload the save if you've been hit.