The "trick" is to use Stand Alone and get your squad back to full health while Treya is kneeling. Without Stand Alone your toons will suffer huge damage even if you try to use healing abilities. You got 5 attempts to use Stand Alone and since she is very fast and hits hard you do not need more because Enrage is reached rather quickly.
It was a mistake to have her inflict an unresistable, undispellable buff immunity because all the toons that survive the first hit after Enrage cannot use abilities that increase damage for a final blow (for example Colonel Starck can not apply critical damage up).