After Samiel's Undeads had carried out their merciless slaughter in the forward fortress of the Human Brotherhood, which had already suffered considerable damage from the attack of Minor Voidborns, which had caused a large part of the fortress to be destroyed, they had a very easy time when it came to that aspect of the battle.
Many of the soldiers present at that time were either killed or injured and were unable to resist the hundreds of Bronze-Tier Undead clad in dwarven-made armors that had been placed around them.
Every step they took resulted in the humans suffering terrible losses. The Undead were moving in perfectly coordinated manner, just as he expected from them, as they employed tactical formation to boost their attack.