Once you examine the mirror, there's no going back for the rest of the game. A long cutscene will culminate in a battle with Xande. Xande will waste a lot of turns casting Libra, but his other Spells are devastating. You'll need plenty of healing to keep your party alive. Keep in mind for this and future battles that Curaja is likely more effective used on a single weak character than spread across the entire party, due to the HP totals you should have at this point.
Once Xande falls, you'll be brought into an unwinnable fight with the Cloud of Darkness. Don't waste any resources, but note how powerful Particle Beam is, because you'll be seeing it again, and you'll need to be able to survive. After the fight, you'll be restored to full HP and MP and be sent to the World of Darkness.
Once Xande falls, you'll be brought into an unwinnable fight with the Cloud of Darkness. Don't waste any resources, but note how powerful Particle Beam is, because you'll be seeing it again, and you'll need to be able to survive. After the fight, you'll be restored to full HP and MP and be sent to the World of Darkness.