First of all, a spoiler warning, as Infinity Gate is a relatively new release. Now onto the calc...
V = 243.84 X 5.18e+6
= 1263091200m^3 (1.2630912e+15cm^3)
Atomization of rock is 30852.2 joules/cm^3.
E = 1.2630912e+15 X 30852.2
= 3.8969142e+19 joules
= 9.3138484703632880013 gigatons
Final Results
Machine Hegemony Destroyer's death beam = 9.314 gigatons
A destroyer from the Ansurrection (or Machine Hegimony, as they're actually called) can atomise everything below it in a range of 2 square miles and 800 feet, including the landscape and subterranean strata. We have our dimensions and our method given directly to us! 2 square miles is equal to 5.18e+6m^2 and 800 feet is equal to 243.84m.
V = 243.84 X 5.18e+6
= 1263091200m^3 (1.2630912e+15cm^3)
Atomization of rock is 30852.2 joules/cm^3.
E = 1.2630912e+15 X 30852.2
= 3.8969142e+19 joules
= 9.3138484703632880013 gigatons
Final Results
Machine Hegemony Destroyer's death beam = 9.314 gigatons