Why Gloom?
A ton of life steal , and that means that you don't need to use Healing Return on melees which lets you use primaries , or even archguns without dying
The slow area , even though it has a very small radius , still works really well when you want to cc a certain target (ahem acolytes ahem) , and because of the negative range , it just means that it barely uses any energy without losing any of the benefits from the lifesteal
The Build
If you feel that you don't need the Flow or the Hunter Adrenaline/[Rage] there and want to have a bit more survivability ,feel free to swap it with an unranked Adaptation or whatever else you would want there
The element choice is up to you ,but I would recommend Ice for the slow procs or ,if you want to get beefy ,just use heat for that sweet 57k EHP
As for the exilus choice ,I think it's pretty self-explanatory why I chose PSF over anything else...
Not much else about the build ,so feel free to watch my build demo if you want to :). It's just a 5 min survival with what was supposed to be an acolyte at the end. ENJOY!