Aliens were fought at the "L5 Campaign", which was the very last part of SRW OG1. So for now, until the story for OG2 gets into gear, there aren't any more aliens (yet).
I feel as if they are pretty much trying to get through the very early parts of OG2 as fast as they can, without losing TOO much. OG2 after all started fairly slow. I think they just want to get to the NDC War and the Inspectors faster. Kinda weird how they had the Chokujin show up that early, when its still just the ATX team lol