Naturally as you know the spiritual tigers had silver grade bloodline. As you can tell most likely they could also use the spiritual or you can say bloodline pressure and right now using that they already take care of those Monsters and most likely at the same time the Boss monster could also feel the pressure coming out from those spiritual tigers.
If you are wondering why, at the same time Allen directly felt a powerful aura not far away from the direction and he was definitely sure that the aura belonged to a monster. So that means it was the Boss monster that was hiding all this time. At this time even though there are spiritual tigers also since the Boss monster but they are busy killing those other monsters so they did not focus on that Boss monster.