This is a video showing off Cless and Chester without their weapons against all three forms of the final boss, Dhaos. Expect a bunch a jump kicks in this video as that is literally the most effective way to defeat Dhaos. On normal difficulty at least, the only form that gave me the most trouble is his feather (2nd) form as shown in this video, otherwise he wasn't too hard overall considering he is the final boss in the game.
Cheats were used to have only Cless and Chester in my party, to allow immediate manual mode in battle, and to have an infinite amount of herbs to immediately max out my hp, tp, and attack power of my two characters. Agility wasn't increased at all by herbs; however, the attack power of both characters were sent to it's max of 999 by hundreds of Red Lavenders to speed up the fight and mainly to be able to damage Dhaos at all. While Cless can freely remove his weapon, the only way Chester can have his weapon removed without cheats is if he had the Elven Bow equipped when he gives the sacred wood from a late sidequest to the elf at the bow shop. This will force Chester to remove his weapon allowing him to run around without a weapon as long as you don't equip a bow back on him.
Without their weapons equipped, Cless is limited to a few artes he can use which includes all the stat increasing artes and Beast, while Chester won't be able to use any of his artes making his TP useless during all three fights.