I think the 'bomb' may just be the IHgauss round, they are hillariously large and bulbuous.
Good spotting with the mystery mech though, I'm guessing Daishi given it may be firing a Gauss rifle, has what I assume are a pair of HLLs in it's arm and has launched a large salvo of ATMs. Not sure if the Ryoken can hold that much firepower.
From that angle though both Ryoken and Daishi would look similar, though the feet don't have that funny outward 'toe' that the Ryoken has as well.