You can unlock Nimbus at player level 20 with coins or buy the suit from the store with real money.
Your method of healing is switching between pistols that deal damage or pistols that heal your teammates, and your big fire mode will spam whatever ammo is loaded in your pistols, so it can either deal damage or heal your teammates. You can also equip the rig that places a healing aura on the field to provide more healing.
Keep in mind, that you do not need to wait for your teammates to be damaged to start "healing" them, if you hit them with healing rounds you'll overfill their health bar and the game will count this as healing. Between fights and anytime you have a chance, keep your teammates health bar overfilled, and when the tank is fighting a larger dinosaur, focus your healing on them. I was able to hit 10k heals after my first match with Nimbus.