Keep killing the rats until the vanguard appears. The indestructible statues may cause your attacks to bounce so be careful. Fortunately, the rats are slow but their numbers can be a problem. After killing a few rats, the vanguard will appear. This doesn't look different from any rats so unless you have AoE attacks, concentrate on that vanguard rat and focus your attacks on him. He's not that tough anyway. After defeating the vanguard, all the other rats will retreat.