Playing on all Normal.

So after like 6 or 7 tries, here's the configurations that made it possible. I lost no party members in either bossfight, and this setup killed both the 4 Chieftains and the final boss first try.

I'm just going to link you to my Imgur were I uploaded the images of the skills my party used.
https://imgur.com/a/kBE1oYK

Other tips:
Put just enough strength/agility points in there to use the best gear, rest into vigor.
I played a Mage as my starter, which I really recommend.
Find all the hidden potion recipes in the dungeon, it helps alot.
Have Mezey Taunt as much as possible, it's easier to heal 1 person than all of them.
Don't forget to equip all the potions you've made.
Use healing potions before using skills that heal to save energy.