When playing as the Sunflower, you can use the heal beam by hitting LB. You'll know it's working when you see a bright light connect you and the player you are healing.
To get this, you need to have the "+10 Healing" bonus show up on the left side of the screen 25 times in one match. The easiest way to accomplish that is to play Gardens & Graveyards mode and focus on healing people really low on health. If they are almost dead you can get 4/5 heals off that one person. Enjoy!