This may be the easiest, best, Shooter Formation in the game at present.
Requiem, Hartwell, Ephraim, Phoenix, and Griffin
These Heroes are fairly easy to get—with the exception of Phoenix—and do not require to place high in events, or win one of the lottery type games.
Requiem here is a Shooter, however her bonuses apply to Troops in general. Unfortunately, there are no shooters that match up in a way that would be beneficial to only shooters outside of the ones above.
Although there are other five star Shooters like Dara, and Nam Hayul these also provide Troop bonuses, and therefore there is no difference between them outside of both these Heroes, opposed to Requiem are far more difficult to get, requiring placing in events to earn the frags.
Importance of Hunt Formation
When you take your troops out hunting either Zombies or Zombie lairs having a good formation with Heroes that match your strongest troops, and the troops that you are dedicating your gear bonuses to, is an important step in maximizing your damage overall.
It is also effective in the Pit, and raids on player sanctuaries. You should always upgrade your troops to the maximum tier that is available based on your HQ level. This will allow you to use your might to its maximum potential.