I cant beat Cereth (Idk if that's spelled right)! What do i do?

Do you have your second party slot unlocked (unlockable in Esior's Haven Plaza)? (third slot is from Sanctuary of Saerielle, so you need to wait for that, but you should have two slots at this point)

Pet elements, proficiencies and equipment are important to pay attention to- make sure to equip pets properly and fully with gear of the right type (attack bonus to the right stat; int> magical element, str> physical element, and agi for a tank, dex for attackers and healers, eg, myrmidon (str/dex) gear for an air pet (physical attacker)), of at least decent rarity (upgrade from uncommon asap) and close to your pets level (equips go by 5s- lv0, lv5, lv10 etc.- you'll be wanting lv15, and failing that, lv10 equips

I don't have slot 2 yet. I started this game 2 days ago.

you can unlock it as soon as you can access Esior's Haven, which you can absolutely do if you're at Cereth