Since the latest update I have gotten back into the Siren and I gotta say I love what they did to Cloud Kill.
My problem with the siren was survivability. I always liked the skills in the Motion and Cataclysm trees but I found myself constantly dying, but then it happened I found a Legendary Nurse Mod. After some tinkering I settled on this build.
http://bl2skills.com/siren.html#505001455051152101004550510501
I use the Legendary Nurse Mod to help fill in the skills in the Harmony tree and wow it’s a huge improvement.
------------------------------- Gear ------------------------------
Class Mod.
Legendary Nurse
Team Health Regen/ sec. 11,900.9
+5 Restoration (6/5)
+5 Elated (7/5)
+5 Sweet Release (6/5)
+5 Sustenance (6/5
+5 Suspension (10/5)
The Team Health Regen alone is not much but Sweet Release (6 life orbs restore 15% of max health), Elated (while enemy is Phaselocked everyone regens 7% of max health/sec), and Restoration (shooting other players converts 36% of damage into healing) further help the team while Sustenance regens 2.4% of your missing health/sec.
Relic.
Bone of the Ancients
Corrosive Damage +39%
Cooldown Rate +47%
I chose this one because Cloud Kill is awesome and the Cooldown Rate is also nice.
Grenades.
Fastball (Corrosive/Fire)
Shield.
Big Boom Blaster
The higher the Booster Chance the better.
--------------------- Weapons -------------------
I use the best I could acquire from tons of farming and trading but with the exception of the Slagga they can be switched for easier to come by loot.
Bekah/Hammer Buster
The Bekah aside from being extremely powerful it is very ammo efficient and works very well with the Siren’s Chain Reaction skill. Also it is nice to have something Non-Elemental for Elemental Resistant enemies.
Slagga
Sometimes your “death ball” will be preoccupied with an enemy far from your location and a lunatic or something will be charging you, just use the Slagga to Slag and chuck a Fastball, The Big Boom Blaster is great for making sure you always have one.
Twister/any shock shotgun
Sometimes the Shock Damage from Ruin just isn’t enough for tougher enemy shields.
Norfleet/Topneaa (Fire/Corrosive)
Any launcher you want really, I just use it when I get downed.
Edit: OP rewritten to look more presentable and Build Finalized, hope you enjoy.