DQ8 boss on Geyzer. both hero and yangus were at level 8, and equipped with the best equipment possible.

I leveled up quickly by training on the slime knight (one knight stand) special monster outside of the waterfall cave, and the mecha mynah
by the mountain. they give a decent amount of exp. for the monsters in the area.

As you can see, I stopped using knuckle sandwich early, because Yangus quickly ran out of magic.