That was not the Forerunner's best ship, it was out dated by 10,000 years by the time of the Flood. And it was tanking the entire ODP defense grid, each firing 51.6 gigatons, rendering them entirely useless. The Infinity is explicitly the most powerful UNSC weapon, equipped as it is with Forerunner technology, and it only punched a small hole in a thin plate section of the ship. The Forerunners regularly employed tens of thousands of ships in fleets, with some of the smallest being designated "planet breakers." The strength of basic Forerunner ship weaponry has been calc'd to be high gigatons to low-end terratons, and have retained the capacity to force stars into supernova, a tactic they used as standard during the Flood war. By no means are even the ICS calcs, which are the absolute highest end calcs that, when put under scrutiny, do not stack up to most depictions, capable of contending with the Forerunner military and industrial juggernaut.