It's actually a clever twist, not a deux ex machina nor plot armor. It was telegraphed earlier in the story and in the anime (i.e. you've already seen her do it). So try to puzzle it out based on what you know.Series is still marked as ongoing.
=> Plot armor ftw
Remember when they were poisoning water wells to thwart an invading army? One of the girls rushes past soliders and jumps into the well with a bottle of poison. Kaoru catches her by putting her in her item box. i.e. unlike other stories, living things can go in the item box. Kaoru realized this could used as a last ditch method of escape in a no-win situation. So she put herself in her item box. Betting on Celes being able to take her out.
