Over these last 2 years, Maria has begun to learn how to control her eyes better than before to help avoid the senses of especially sensitive people like Mira.
She has a pretty good idea of just how sensitive Mira is to stares and it could be said that not only can she determine a person's intentions based on their stare, but she can also determine a lot about a person based on that as well.
So if Maria stared at Mira with the intention of joining the battle, Mira would definitely be alerted. To combat this, instead of watching Mira closely and waiting for a mistake, Maria decided that she will watch Celaine and attack Mira based on where Celaine is leading her.
She slowly made her way towards Mira's blindspot and prepared her attack. She was going to use her fastest move which just so happened to be the simplest, Light Beam. Basically, she's just going to shoot a supercharged beam of light at Mira and just hopes she can catch her off guard.