This trophy can be earned in all modes except tutorial.
The easiest mode to do this trophy would be ambition.
Set the difficulty to easy & have a decent lv character.
Pick a skirmish.
Run straight for the enemy leader & defeat them & their surrounding mobs.
Once complete the trophy should be yours.
Extra Tip: Try heading to the leader with full musou if need be. This will allow you to spam your special and clear the unnecessary mobs with ease.