Do you mean in between your pressure? Well, Mileena can't just throw EX roll, she actually has to make a read, she uses mostly to escape pressure, to mix you up, or as a wake-up. As a wake-up, I suggest you to use an armor breaking move if your character has one to make her respect you, then start mixing her up if your character excels on that area, or simply block, but you will have to stay guessing the whole match against her. On pressure I suggest you to bait an ex-roll and punish, and same happens, you make her respect your pressure a bit more, althought ther's always going to be a treat from EX roll.