Well, I have a few ideas to help even the odds slightly but not enough to take it down to a fair fight. Or even slightly lopsided.
[@Wildman] "Soon the experimental Sky Piercer cannons were finished and were hauled into the underground tunnels placing them in key strategic locations underground and with their crew with them, they are tasked to produce or procure weapons for the time until they were needed again, soon these monstrosities of iron and metal were hauled into the tunnels fading into the darkness by dragged by equally monstrosities the Rarians where able to conjure in their time of isolation."
I have multiple problems with this. For one, you never mentioned such massive guns before, in fact, you barely got into producing your own small arms. Understandably, I don't see how this convenient equipment happened to be convenienty finished on the day of the EOM assault. Second, and this is less of a GM and more of a reader advice, the following sentence is almost 4 lines long, and yet it is a single sentence. Sometimes I struggle to understand what you are writing, because you connect so many and words to make these monsters of a sentence. I am not the best writer either, but I think you should make many small sentences instead of a few of this enormous ones.
Back to then guns, I definetly don't like this ideal turn of events, and guessing by the name and description, you intend them to be capital busters. That is a big no-no. On the other hand, I can see you guys stealing the smaller EOM space guns, and perhaps making similar replicas, but definetly some big ass OP guns. Perhaps if the siege gets prolonged you could make ONE, but not multiple sites spread across the planet.