This is the method that got me the achievement:
During the second half of the fight (after he reveals his heart), take control of Raven. Make sure you don't have any weapons or skills equipped that will alter your artes (you want normal Rain, not Rainsong). Keep your allies at their normal strategies and back up, making sure Yeager is facing you. Now go into Overlimit (having synthesized the Quatro for level 4 will help) and simply spam your Rain arte over and over and over. Your allies should add in some melee strikes and eventually he will go into a hunched over state where he is catching his breath. Since you are spamming Rain, it should have no problem hitting him directly after he does this. Since your allies are up close, they may hit him first, so use some Limit Bottles and try again until it works.
This youtube video seems to be an easy way as well, but I never tried it. Watch the video and click "more info" to read about his setup and exactly what he did.