I can't defeat him. I'm using level 17 Sylesti's i spent all there level ups and proficiencies but i still can't beat him.

Well Zeus could really use a new relic. It looks like you have him geared towards strength which does physical damage, but his relic does magical damage. Since you can only do one type of damage, that relic is a waste. Also you might try scrounging around for some more level 10 armor....

Can I ask what proficiency your pets are and where you put their stat points?

I don't remember the Cereth battle very well, but I'm assuming you can't get your third party slot yet?

Also don't forget to use your finishing move when you have built up some mana!