That may taken from movie, while the shield idea is really OK, have you seen how many reactors have even the Diablo, I think that was they have multiple ones, to stop someone from making a suicide maneuver, or even to self-destruct in hey here "Independence day" where you see a signal "Big Alien Ship, weak spot 10,000 meters ahead".
This also goes to Reactors (the Gigas has 6 Reactors, the Diablo has 4), Shield Generators (the Diablo has 2) and Gigas's Stabilizers (it has 3), and many other systems which are spread in order to have a preventivy SubSystem which doesn't lead into the collapse of the Ship (Gigas's Stabilizers) or the Shield systems (Diablo dual Shield Generators)
The Armageddon or Executioner would a best choice in any case, by the size of the Gargant, it should have a really good Power Output for Shields and Beams, that means that if the shield output is just used to protect a single big reactor, it would be a miracle that even a Helios could make an scratch on it.