Can you use astrea ability during your attack if you have only one creature and attacking with it? IF yes, I guess it will prevent your creature from attacking? but you will be able to summon a guardian
Pretty sure, yes. If you have attached the Divine alteration, you can move the exhaustion token off the single unit though. Or, you can pay a basic to put the exhaustion token on the Guardian when it comes out. I think that’s how all of that works. Plenty of options.