She didn't know the power she had in that moment, she was facing with the best she had without use Crystellar, at the time she heard Lilith advice and took Crystellar on hand the battle was decided, the Gospel Weapons are super strong against Ancient and Evil beings and the curse of Exodia was Ancient and Evil type the attack from Crystellar was ultra effective for that reason.
With Crystellar on hand Meriana is amazingly strong and that's not the full potential of the gospel weapons im sure they get stronger too *nods* and Meriana is just an amazing warrior now she became a target for Exodia having release Aidan from the curse.