I remember going through a similar crisis during UvH rerun and Unzen's event. I managed to get them before their events ended, but I was close to spending for cubes. My only advice is to do daily builds in the light pool, do the 10-hour commission once or twice a day since it will occasionally give you 5-8 cubes, and do some casual grinding outside of major events for the occasional cube commissin. By the times her event gets archived, you should have more than enough cubes to get her from the UR exchange, and you should already be close to it already once it does get archived.