Someone help me plz i cant defeat her and her knight and my greatest sylestias are level 27 and 25 and i have armour on them someone help me plz plz and thanks

omg... I have the same problem exept I'm in the level 40s ...XD I feel sooo useless and like I'm really bad at this game...

How many pets do you currently have in your battle party?
If you're able to expand your party maximum (in the Plazas in the towns) definitely do so.

I see you have DarknessStormLightening in your party and it looks like you've spread his level up points quite evenly across all the statistics. It's better to focus on a handful of statistics and get appropriate equipment and proficiencies that then also enhances those chosen statistics. Here are a couple of useful guides on choosing statistics, proficiencies and equipment: here and here.

Otherwise, when you have a multiple enemy battle, it is imperative that you take the strongest opponent down first, then the second strongest, then the least strongest, etc. You can tell which are strongest by the number of stars (in the battle you're currently struggling the Arch Mage will definitely be the strongest). Flute created an Illustrative Guide here which has graphics on multiple opponent battling in the first post.

Good luck!

Can you help me too? I'm soo bad at this!!!!

If you still need help, I took a look at your party pets. It looks like you know the basics to assigning points, but you should definitely fully equip your pets, and try to get better equipment for them if you can (legendary or mythical is the best, but might take a while to find and may be expensive if you're buying).

The guides Wander linked in the post above yours are very useful, and you should look at it, if you haven't already.