I...I just.... I just can't beat Cereth. What's wrong? Do I need a certain type of armor? Did I choose the wrong starter pet? Help!?

evry starter have the same base stats, you may nead to get better or more fitting gears for your pets. Keep in mind the stat that are good for his element and try to bost those.

I'm sorry, but what element is Cereth?

Cereth is not bonded to an element. I believe Cian is referring to your battle pets Axel and Night's elements and gear.
Axel is level 22, but his gear is level 10 or 5. Same goes for Night. You should attune Night to an element as soon as possible; I recommend Wind. You'll want higher-level, higher-rarity gear that boosts the stats each element uses (for Axel {Shadow}, Intelligence and Agility; for Night {going with Wind}, Strength and Dexterity).
It's impossible to choose a "wrong" starter pet. Some pets are more inclined toward certain base stats than others (for example, Sylvorpas have average all-around base stats, while Ny'venes are very Strength-based), so you can build up a party from the base that way, but in the end it's really just looks.

Thank you so much! Wish me luck!

Your luck has been wished! And if you can't beat him right away, remember to stock up on revival potions and elixirs to heal in battle for next time!

Wait, i thought all the potions couldn't be used in battle. And If they didn't say that, they won't show up in my inventory.

Potions cannot, no. Revival Draughts and Elixirs, however, can. Draughts are green and have round glass containers, Elixirs have triangular containers. Potions are rectangles.

Okay, I have a ton of those, but none show up in my inventory. Why?

Have you tried the pet refresh during the battle? That's where you can access the elixirs and draughts.