Adds > marshall > boss
When marshall and adds spawn, just kill weak adds and focus marshall, and when marshall is dead just kill adds (soldiers are quite weak so its quite easy kill them with aoe spells same time when damaging marshall) and then again go for the boss.
Do this and boss should be dead after a while.

Tip. if you have black mage in your party, tell him to use limit break on the first wave of adds or any soldier wave, so you can focus marshall faster.