Dundoth may seem to be an easy boss but he has to be treated with caution. He has a lot of attacks that can hurt the party and slow them down.
The most important thing is to take out all the bandits that are assisting him or this fight will end up badly. Onyx Wave will finish them quickly but it will burn 1/3 Graham's HP. With all the bandits gone, it's time to finish the boss. Slice n' Dice does about 300 damage, Roundhouse inflicts a bit more that that, not to mention Stun. He also counters physical attacks with Whack, dealing 500 damage and confuse. So let the Summoner, White Mage or Red Mage attack the confused character. Also, Thwack can damage the entire party for about 200 Damage. Whispering Wind and Cura are essential there. Try to avoid physical attacks by using Strike, Enrage, Aurablast and Darkness. Make sure the party has a main healer. Don't try to rush all attack at once. Since the player has beaten Hecatoncheir before, this guy can be handled with not that much effort.