A few ideas for that mission -
Instead of just mission fail if you hit a hidden firstborn, let it transform and make it invulnerable to Cole, and make Cole take one-hit-kill damage from it. This would probably add to the sense of peril in trying not to hit a firstborn.
Instead of a crowd of people and one is a hidden firstborn, have two crowds, and one crowd has a firstborn in it. It'd also be good if the firstborns were added to a randomly chosen crowd each time the mission starts, so the player still has to guess.