Resolving dice allows you to resolve all dice of the same symbol one at a time. So say I have three dice showing ranged damage. Two black showing 1 dmg, and one showing + 1 blue dmg.

Can I play Supporting Fire card increasing one black to 2 and adding the +1 to the other for a total of 4 damage in one action? I believe this to be correct because of the way resolving dice work

Supporting Fire lets you resolve one of your dice. So when you take a action to to exhaust the support and do it's ability, you can only resolve one die at that time. It is not the same as just taking an action to resolve any/all of your dice with the same die face.

Okay I agree only because it states Action. This card is effectively dead as it stands. Especially against fast hands and Force Speed.

Yup, this card is terrible. It might see play if you could resolve all your ranged dice, but even then, probably not.

However in the rare case that that one extra dmg is all you need to kill a character, the card would actually be good so you don't have to use a second die that may be showing more than one dmg and waste the extra on overkill. But still not the greatest of abilities.

It works best if you're playing a game where your opponent keeps claiming the battlefield. If you just know they're gonna claim, you might as well take your time and get as much damage through as possible.

I think there will, at some point, be a place for a support heavy deck which slow rolls out, lets the opponent claim or take all their actions, allowing the player to hit the enemy for damage.

In such a world it may be possible that supporting fire finds a place.

The problem with Supporting Fire isn't that it's slow, or that it's a bad effect. It's that it costs you two resources. It's never likely to be viable for that simple reason - it's horribly inefficient for what it does.

Reprint the effect at 0 cost for an effective + 1 damage a turn, and it might actually see play.