The fastest way to do this is by choosing the material battles as you can easily take out the main officer in less than 2 minutes.
I did it at level 30 however the enemies had massive HP bars during battles 10-20 so you may have to run past everything and take out the main target. The most effective way to take out the officers during the latter battles is by using a true rage attack so if you have a weapon skill that helps build your rage meter this should be a breeze.