A Monk is a well aggressive character! I sometimes have him as a Dark Knight, but he seems to do more damage as a Monk. The See You In Hell ability is amazing. At one point in a battle I just either used Phoenix Down/Raise on him but didn't use any potions/magic on him afterwards, so he would get knocked out and do about 8-9000 damage to every enemy.
Isn't Obliterate an attack which does the dark attack?
I've found the Valkyrie skill Decimate quite useful too. Taking off a fair chunk of effort with the results.