Okay, I've taken a while to get over my initial annoyance with this question, since it's not you're fault I'm annoyed. This is going to be a long response, so my apologies, I'll try to add a TL;DR to the end.
First off, this critique advice "It doesn't fit with x at all" loses value when you realize 1. I am rewriting the games story and thus the only solid part of Guinevere is a 7 year old girl who appears in one or two scenes, and 2. That it's completely in character for the Guinevere of this game.
Let me explain.
Guinevere's character is the traitor princess, who betrayed her own country for the good of the world. She, as she says in her supports to Roy, is a liar when she needs to be, she lied to him when he was at his darkest because he needed it to survive. It tears her apart to do it, but she's willing to do something she doesn't want to if it means stopping Zephiel, for goodness sake, becoming a warrior is one of those things she does not want to do, she does not want to fight Zephiel, but she must, because he must be stopped and she cannot stand in the back waiting for others to do it for her. She must bloody her own hands if she is going to ask others to as well.
Next, Guinevere is very politically minded, she knows how to talk and how to act with people, it's a good trait, it's what she brings to the lord trio, mixing Roy's hotheaded nature with Lilina's good natured heart. Now lets look at the design of her promotion. It uses light magic, the magic that is seen as holy, and she uses the lance, a weapon that is traditionally seen as the weapon of the romantic knight. Also associated with the traditional fantasy Paladin, along with the Warhammer, Sword and Board and Greatsword. She is dressed in full plate, which gives her the look of a lady of war. Her design is built from the ground up to resemble a holy hero.
Lets look at the crit specifically, she uses her light magic to create wings, giving off the angelic vibe, before slicing twice with them, and then doing a diving kick (a traditional hero move in fiction) to the impaled lance. In short, the entire crit is built to make her look as heroic as possible. In short, from a character perspective, Guinevere is selling herself as a hero.
Let's ask why... Well when the war is over, she knows she's going to be the queen of Bern, and that the rest of the nations are not going to look at Bern well. But if she takes the role of the liberator queen, and the word spreads of this holy warrior who resembles an angel, heroically fighting on the battlefield, then Bern will be in a better position.
This is counter to Roy's Crit, which is a focused series of strikes going in for the kill, and Lilina's, which is focused on her magical nature. She also hybrids the two. Lilina uses her magic exclusively in her Axe crit, Roy forgoes his until the SoS crit, but Guinevere enhances her self with the magic and combines them.
So in short, this crit fits perfectly with the tone and arc that I have written for Guinevere, which still encompasses the major plot beats of her character in the original game. While not being a slave to the fact she's a sage if you beat the game 10 times and at no other point is showcased properly.
Frankly, the adive "This doesn't fix x's character" is as useless as going to Arch and saying "Hey Raven's story doesn't fit what I think Raven would do." While you're technically correct in the purview of what FE7 itself presented, the idea had for the character and their reactions is entirely up to the creator. It would be like saying the soldier animations for Dorothy don't fit the character because she used to be an archer. This was true, while she was an archer.
And please, do not take this as me saying you're not allowed to tell me you don't like something in the writing or the presentation. But I need substance to critiques and relevance. Thank you for posting your feedback however.