Well, that clears things up.
The reason you're having so much trouble with your Serris is that her gear is flat-out awful. There are three main issues with it.
1) The set choice. Perception is a good set choice for her, but attack is fairly pointless snce she's not a damage dealer. Most serris builds use a mixture of speed and accuracy/perception gear since those are her two most important stats. (I would advise you take the other builds mentioned with a grain of salt. Immunity is wonderful in arena, but not so much in other areas and serris just doesn't have the hp to make a shield set worthwhile, unless you gut her main stats.)
2) The rank. Having rank 5 gear on her will make a huge difference. If you don't have what you need in the right sets, it may be worth it to put her in mismatched gear as a stopgap. You'll lose the set bonus, but the higher gear increase should ideally make up for it. Particularly in her def stat, since going from rank 3 to rank 5 is a pretty significant step up.
3) The level. Your gear is level 8. This is certainly your biggest issue as to why she's not performing, since all her equpiment is providing less than half its stat boost. Obviously, getting all her equipment maxed will be pricey and may have to wait, but try and at least get everything to 12 (weapon can be left as is for now since her atk really doesn't matter), and get the most important ones to 16 asap.