i found that i can't get past the cereth boss battle as i cant use health potions in battle. this is in fungus grove.
if the problem isn't fixed, then i shall have to quit sylestia. thanks, Oceanwarriorcat

Only some potions can be used in battle. The game will tell you which potions cannot be used in battle. Stock up on potions that can be used in battle to help you survive.

Also, your party consists of two tanks. You should switch one to a damage role, fire or air will do that (I recommend air).

Finally, make sure you use equipment that boosts the right stats for each pet. An air pet should have strength/dexterity for example. Check the guides for what stats are important when you do elemental attunement for pets (except light pets that is), and don't put health on any pet except a tank pet.

Potions cannot be used in battle- In battle you can use Elixirs to restore HP/Mana, and Draughts can be used to revive pets.

Also, looking at your pets, just some notes that might help with the fight:
Typical Tank elements (pet in slot 1 of party) are Earth, Water, or Shadow,
Healer element is Light
Glass Cannons are Fire and Air. (Other elements are less oriented towards damage dealing, but Shadow is one that can definitely do so reasonably well)
(As a note, the Cereth may be best fought with a different combo than Earth/Shadow, as Earth is perhaps more effective after it gets its level 30 ability. The Sylestia Wakthrough suggests maybe a Water or Shadow Tank, with a Shadow, Air, or Fire attacker for the Cereth fight)

In this case, your Tank would presumably be claw, and your attacker Galaxy?

Name: Claw
Level: 13
Element: Earth
Prof.: 1HP
Relic: Int/Dex/Agi, uncommon, lv5
Armor: Int/Dex/Agi, uncommon, lv0
Acc.: -
Acc.: -

> 1HP is okay as a prof. on a Tank.
> Uncommon equips are, frankly, terrible. Example in the fact that a level 55 uncommon equip has about the same stats as a level 20 legendary equip.
> Earth pets run of Phys, and as a Tank, the pet wants Assassin (Str/Agi) gear.
> You wanna have all the equip slots filled.

Name: Galaxy
Level: 17
Element: Shadow
Prof.: 1Int
Relic: Int/Dex/Agi, uncommon, lv0
Armor: Ravager, epic, lv5
Acc.: -
Acc.: -

> As with claw, you want to fill all the equip slots.
> 1Int is fine for a Shadow Attacker.
> As stated, uncommon is Terrible.
> lv0 is also bad- generally you want to keep the equips within 10 levels of the pet.
> Ravager equips are more for Strength Mission Pets, rather than pets used in fighting. More than that, Ravager equips are focused solely on the Str stat, which isn't going to help a Shadow pet, as they use Magical Damage, shich runs off of the Int stat.
> Shadow Attackers should be given Evoker (Int/Dex) equipment.