Hi can you please give me some suggestions for how to defeat Cereth? I have tried multiple times but I still can't get through... Please tell me!

I'd suggest using a Tank element (Water/Earth/Shadow) and an attacker element (Air/Fire) rather than what seems to be a Light pet as Tank (can be done, sure, but not the typical) and a Shadow as Attacker (can also be done, but not as oriented to it as Air and Fire)

And Equipment. Having the right sorts of Equipment on your pets is pretty vital to sylesti, especially the Relics, which is where the bulk of a pet's offensive power comes from. Not having any on your pets is a serious hindrance to both their strength and survivability.

Equipment gets stronger based on both the level of the equip and the rarity of the equip (try to avoid using Uncommon equips (green) when possible because it's garbage, but even it is better than nothing)

Water and Earth Tanks want Assassin (Str/Agi) equips

Shadow Tanks and Light Healers use Illusionist (Int/Agi)

Light Healers and Fire Attackers want Evoker (Int/Dex)

Air Attackers should have Myrmidon (Str/Dex)

Ah I remember that well! Its all in the equipment and determination. Stock up on healing potions beforehand and make sure that your armor is the right kind for your party pets. If you have only one pet in your party, make it two! And just keep on going until you get it. I got so frustrated that I stopped trying for a week. Then I came back and did it again, and got it on my second try. Good luck!

I kinda winged it, but I did make sure to have the equipment that best suited my pets. XD And health potions. Lots.