How would I go about making an emitter that deals negative damage?

You mean you want something that heals? Check out the code for the rejuvinator in soupiest's Combined Forces, is my recommendation, because you need a special script and lua to do negative damage as far as I'm aware.

Well, technically you can do it, but emmiters as in wounds.
If you change the EmissionDamage of a wound to a negative number it will the target.
But, as I suppose, if you want to do something like a healing gun you need a lua script, which is one of the most simple scripts out there.

The negative value of an AEmitter way means you can't put it on an attachable of the actor, only on the actor itself, Otherwise it won't count towards the actor.

Fortunately there's this:



So giving something like this to your attachable might be what you want if you want negative Emission damage.