The Arkham soldiers might have never thought that they would end up like mice caught in a trap waiting to be slowly devoured by the predator.
Initially, they thought that they had a numerical advantage and using torches and other means, they could find the enemies but their enemies aimed exactly for the ones carrying the torch and then engulf them in darkness
No one was able to spot Alex even if they relied on the light torch. Meanwhile, if Alex's men were spotted, the watcher wasn't left alive to spread the tale.
And the whistles used to alarm others, instead of alerting them created more confusion.
The Arkham soldiers were frustrated and pissed off. But more than that they were scared of the unknown.
Overconfidence caused them to commit careless mistakes. They should have never stepped in here during the night.