We're already trying to be stealthy as -blam!-, why do we have to be lightning fast as well? The fact that it happens sooner and sooner each time a beast is killed just makes it worse. It gets to the point where you leave the room and have to head back inside almost immediately.
You're already giving us limited uses of the safe room, why are you giving us limited time outside of the safe room as well?
This seems like something that should be part of Prestige Mode as opposed to Normal Mode.
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Just lower them all in the first 2 rounds and kill them in the 3rd.. if you screw it up you still have the 4th to fix it.