Easy way to destroy it...
Launch a few dozen Titan-II missiles, currently in the US military, at it.
Would devastate anything smaller than a thousand kilometers.
As the blowfish said, the Lucifer
can survive an explosion of 1,250 gigatons
. I bet the Gargant could survive several hundred teratons.
(show your working:
Harbinger Damage: 3,200 (5 gigatons)
Lucifer Hitpoints: 800,000
800,000/3,200 = 250 Harbingers (5 gigatons each)
250*5 gigatons = 1,250 gigatons of damage.
Oh, and BTW:
Gargant Hitpoints: A LOT MORE THAN 800,000