In order to Regenerate HP with Ragna, you must utilize his drive attacks. These are used with the
button by default. If you play as Ragna, using these moves a ton with eventually net you the achievement. His overdrive boosts the effect of these skills, but it's so brief that you may as well use Overdrive and then use either of his distortion moves. There is a faster way in local versus, however.
The way I did this was I moved in to the corner while playing against a second controller and simply held Forward + A. I kept mashing this, even while the combo went blue and I still got progress for healing. I let the combo drop before every round ended just in case, because in some instances I noticed my achievement progress didn't update.
EDIT: IHeartParamore has confirmed that while the damage of your combo scales, the health stolen does not. This means you can keep spamming a "blue" combo and earn rapid progress towards the achievement.