After Luan's barrier cracked, chaos ensued; the monsters' cautiousness completely disappeared as they all dove headfirst at the humans now without their damned wall. That made them basically sitting ducks.
All the soldiers at the front line, together with Dimitri, saw one monster after the other jump over the river without a care in the world. They saw the monsters that had jumped before them get pierced or cut in half, but they still kept going.
Like that, the once one-sided battlefield started shifting as the monsters were slowly overwhelming the soldiers on the front line.
As one monster after the other was able to land across the river, they started to wreak havoc, attacking any human in sight. Tearing a piece of flesh from legs or, worse, biting off a hand.