"The enemy suddenly let down its guard!"

I believe that the last hit dealt a lot of damage to the knight. Probably something like 1000 damage instead of 170 because of the very high defense. 

Because of this, the knight probably got its armor damaged as well as the sword when Susie chips it. The way the knight seems to scream and pulsate like something is wrong going on, the knight was genuinely taken off guard by the last attack in a fight they weren't even focused on. They had to focus on blocking Susie's attack and they were able to take us down because the knight fixed its mistake and when Susie and Ralsei let down their guard, the knight attacked them.

As we know from Undertale, if you let down your guard, you get a lethal attack as we saw in the sans fight.

And this seems to work in Deltarune too as the King took everyone down when they trusted him. 

What does that mean? 

In a future chapter rematch.

If we can somehow manage to get the knight to drop its guard... The win might be possible.

Here, in chapter 3 the knight genuinely didn't expect a real fight.

Makes me believe that when Kris coughed and the knight tilts its head, they probably were confused making them drop their guard here too.

Seems like the plan was just to decimate the heroes in one or two turns

Then, kidnap undyne. This defeat wasn't planned.

Tl;dr 

We use some acts to get the knight to let down its guard in the rematch to win.

This might be the single most important line in the game. And probably how we'll defeat the Roaring Knight.