The first thing you need to know is I will be saying the 5 star version of any thing I am talking about. If you have 2 alchemers you can do this shoot with the first alchemer then switch to the other then shoot repeat again and again.
Also another technique is shoot shield shoot shield and repeat.
If your going to do FSC you should use this as your loadout:
Perfect mask of Seerus
Shadowsun slicker
Swiftstrike buckler(if you run into fire a lot you should bring a GOS
Hail driver
5 star prismatech driver(or maybe Polaris if you want)
Blitz needle
Argent peacemaker
If your gonna do RJP use this stuff:
Magma driver
Umbra driver
Biohazard
Any gunslinger set would work fine(best would be justifier though)
Swiftstrike buckler
If you are going to attempt RT the best stuff a gunner could use is probably:
Perfect mask of seerus(or nameless hat)
Nameless mantle
Swiftstrike buckler
Magma driver
Polaris
Argent peacemaker
Shilvermist buster
Bump