Reactor Weakness
Not sure why the rebels were surprised that a reactor core was a weakness... reactor cores are always a weakness in every large vessel, and they tend to go critical when systems are disrupted.
It would have served A New Hope better if Erso had stated a small thermal exhaust port--right below the main port was the weakness in the Death Star Battlestation, not "I have built a weakness in the reactor" um what?
Every reactor has a weakness. Just look at the Trade Federation hangar bay, the idiots actually placed their reactor in it.