For this achievement you have to defeat Barachiel in the engine room of the Celestial Ship, on the second planet.
Levels in the upper teens for characters should be enough for you to handle the boss's attacks.
As seems to be the case with meteorite related bosses, most attacks will do almost no damage whatsoever. The weak point for this boss is the small orb behind it's spike that it drives into the ground.
Controlling Edge, wait for it to jam the spike into the ground and then rush in and unleash a skill, preferably a ranked up Stampede Slash. You should knock him back as you do this and if you're lucky you can chain it into another combo for more damage.
If the boss does this in Rush mode (glowing), he'll not be knocked back and will continue with his attack. If he does this, just nail the first combo then retreat back outside of the range of his attack.
Should be fairly simple as his attacks don't seem to do a ton of damage that can't be healed quickly by Lymle or an item.