I spent 2 runestoners only to get the Phalanx, which changed her usability a lot more than I expected. Well for one, my Phalanx are level 9 or so, while the other troops she could use are level 2 or 3 in my book, so of course that didn’t help, but the theme of the Legend of Heroes character is that their troops are only buffers: they are not meant to go into battle. Red Horse Cavalry is a necessity for Leonhardt, as Phalanx are, in my opinion, to Estelle, as she wasn’t able to sustain a single hit before, and she has that tendency to rush into troubles because of her talent. Now it takes an angry Lana to take her down in one shot, and she might actually survive quite a few tanking sessions if no Elwins are involved.
She might be viable in other ways as a real tank, but that’s not how I use her.