Vs. Yeager, hit him in the heart with Raven's Rain move when he's reeling.
YOU DON'T HAVE TO ACTUALLY PLAY AS RAVEN TO DO THIS!!
Take Yuri and Raven with you. Put the other two players on defend, so they'll stay out the way. Turn off all of Raven's artes except Rain (時雨), and set him to use artes only. All he'll do is shoot Rain continuously throughout the fight. Now just beat Yeager down with Yuri. When Yeager moves to his second attack pattern (after telling you he'll show you his real power) keep using artes on him - Destruction Field (爆砕陣) is good as it's short and keeps you near him - and eventually he'll stagger, at which point Raven should shoot him for you. 'Great!'
As with all Secret Missions, the achievement will pop after the fight.