The effectiveness of the Heal/Hurt Gun is controlled by Rate of Fire Endorsements. As it is a constant stream and not individual shots, Rate of Fire Endorsements can increase your damage per second quite a lot. In addition, Critical Shot endorsements help improve the chance for the Heal/Hurt gun to critically heal or hurt in Blitz mode. The Heal portion of the Heal/Hurt gun will never critically hit in Crossfire.
You can build up some Juice with this weapon to heal and hurt allied and enemy Pros, Bots, and Turrets. Don't just stick to one person/object unless no one else needs you or your team is doing very well; heal everything and everyone that is your team color. They will thank you later.
Since you're a walking health machine, you're going to be a big target. Assassins are one of your worst enemies. A single upgraded back-grapple means instant death as does one from the front if you don't have upgraded Armor. If you see a ripple in the air, pull out your Shotgun, as you can't use your Heal/Hurt Gun on her when she's Cloaked. Note that while Assassins cannot be hit with the Hurt Beam while cloaked, Gremlins can be hit with it.
Push with your teammates! If you and someone on your team have full Juice, charge in with them. The Heal/Hurt Gun is lethal when you have full Juice, able to destroy Pros and Turrets faster than almost any other class, and since it doesn't need to be aimed, you can "fire and forget." This makes a Juiced Support one of the scariest Pros you can come across.
Taking an enemy down is not impossible, and the fact you gain health means it can be hard for them to kill you. However, even with Rate of Fire Endorsements, it can still take a long time to kill someone, meaning they can kill you, or someone else will kill you, or you might have your kill stolen. It is much better to use your shotgun, as it does more damage per shot. Only use the Heal/Hurt gun if it is a weak class (eg. Sniper with no health endorsments), someone who has already been damaged, or if you are healing someone and you do not have time to switch weapons.