In regards to Galactus' durability weakening as he gets hungrier, read God Hunter. Surfer says that Galactus in his starved state is "vulnerable" to the fleet that Bill was leading.
Surfer: [talking to Bill about Galactus' starved state] "As he is, he is vulnerable. This slight break in feeding has weaked him catastrophically, if his enemies fall upon him before he feeds..."
The bolded clearly indicates the fleet would probably kill Galactus if they attack him before he feeds. Surfer also mentions if Galactus dies all life within forty-two light years will perish.
*edit: Too add further they also mention how Galactus' core reservoires of energy that consist of his being were depleted due to what happened to him in Annihilation (when he was turned into a weapon). So as far as I know Galactus is currently much weaker than normal due to what happened in Annihilation and he's trying to replenish those core reservoires of energy he lost. Not sure if that extends to Chaos war Galactus but thats food for thought.