Chen -really- needs investment in her comparatively low attack stat to help get through the enemy's defense enough to still deal worthwhile damage. Unfortunately, the abundance of high-defense bosses later in the game makes things rather tough for her, but if you're using stuff like Yakumo team and/or def debuffs she might still work out pretty well!
And this is why I'm wondering why the developers didn't just give her Piercing Attack, since her DPS won't have to suffer so much if some of it can tear through regardless of the enemy's defences.
As for what Chen could use for a subclass, as she is now, I would actually suggest Monk, due to the spellcards that it grants, rather than the skills. I can understand why the Warrior skillset can help Chen out, and also why the Gambler's skillset would look attractive once you actually get that subclass unlocked, so...
As a side note, has anyone tried the Warrior or Monk subclasses on Komachi? The row strengthening skill from the former would help power up her Cold element spell, and she would gain access to the Fire element through Explosive Flame Sword. The latter would give her Wind(buffs speed) and Nature(Pierces defence) attacks to work with, and Body Revitalization has a 4% HP regen which can only help Komachi in restoring her HP alongside her Regeneration skill.
Anyways, on a separate note, I would think that Guardian would be more useful if the following was added to it...
1. The passive that grants a DEF Buff would also give a MND Buff as well.
2. The passive that reduces the time used up by using the Focus command was granted a increase in Max Level, so that the timebar would be set to 8000 instead of 7000.
As it stands now, I believe that Guardian was indeed built with Momiji in mind, since she can counter the drop in speed with Accelerate, and her Eyes that Perceive Reality skill can serve for offence and defence by negating the enemy's buffs. Add to that her accuracy increasing skill, and it's clear that the subclass was meant for her. Plus, the whole "Momiji has a shield" sort of thing does seem to help thematically speaking...