Well Kimo dreadnaughts have main guns that rival those of a reaper in strength. (Improved Turian Thanix guns who were based on Reaper technology) Even if the Krall supership can withstand a blast like that (which it probably does given its sheer size), it won't just be one shot. It'll be constant bombardment from multiple Kimo ships. They'll take their time, rotating ships if need be since heat buildup is a severe limiting factor in space engagements. Speaking of heat, if the bombardment is long enough, even if the Krall ship somehow stays in one piece, it won't be able to vent heat fast enough, effectively cooking the crew alive... I guess in short, the Krall supership doesn't stand a chance on its own regadless of how tough it is (and it's very tough I gather ;p). it'll need reinforcements to try and drive off the dreadnaughts if it wants to have any chance at survival.