Felona wasn't a fool to not understand what Eren was implying.
In this situation, all the Spirits were useless. The only person other than the Necromancer who could still fight was Eren.
She didn't know if Eren could actually defeat the Necromancer, especially one who was this strong. Even she could barely stop his spells while protecting her people.
Despite that, he was their only hope at this moment.
She didn't place her hopes on Eren beating the Necromancer, but she didn't need that in the first place. All she needed was for the Jar in his hand to break.
"Break that Jar. If that happens, we should be able to stop him."
Once the Jar was broken, she believed that they could take their fate in their own hands, instead of relying on others.
Unfortunately, it was impossible for the Spirits to break that jar as all their attacks on it were useless in the first place, and it was even worse in the absence of spirit essence.