So you're at Serra's estate? Right. Serra Voclain was once a famous model, so she's all about beauty and reflections. Her gym mirrors this-- literally. Serra loves to play games with her mirrors, increasing evasion, lowering accuracy, and even performing critical attacks from hidden places. In the Mirror Field, evasion and accuracy shapes effects the strength and frequency of critical hits. Be careful not to take a surprise attack!
But if it happens, use that moment to strike! Her Pokemon has to come out of hiding to attack, so those evasion buffs will be lost. Besides that, consider using beam and light-based attacks! These moves can be reflected by the mirrors, and will always hit!