Your Karma meter has to be a good chunk of the way towards the light side in order to use this ability - kill Blood enemies (the ones with red circles under them when you target them with L) to charge it. Once your meter is well into the light side, and once you've taken at least 25% damage and are in an area with lots of enemies (3+ is good), tap on the Karma bar on the touch screen, and select the symbol that's at the very top right.
This will activate the Heal Me ability, which is indicated by a little white circle on the screen. Use the Vita's back touchpad to direct it, and keep moving the circle over the enemies on the screen. Each time the circle touches an enemy, it will leech a little bit of health from them; the trophy will pop once you've restored 25% of your health bar. The ability lasts for about 10 seconds, so if you don't recover enough health the first time you try, just refill your Karma meter and try again.