EDIT 4/25: I finally got a second Sevagoth to feed to Helminth and will be testing Gloom in the following days, I think I'm going to be switching to it over Breach Surge.
EDIT 4/27: I have switched to Gloom for my Helminth ability, it offers CC, healing and a damage amp. All things that Saryn wants. I have updated the build to reflect this.
The ONLY game mode I don't recommend using Gloom is Defence with a group. You'll simply slow down and piss off your teammates!
Quick and Dirty DPS Build
One thing to note: This build is EXPENSIVE. 198,090 Endo is nothing to sneeze at.
You can find my Beginners Saryn Build here
And my Intermediate Saryn Build here
Growing Power, Transient Fortitude and Umbral Intensify for Good base damage, faster damage scaling and a high slow percentage on Gloom.
Augur Reach, Stretch and Cunning Drift for high range, making spores spread all over the place and giving Gloom a sizable AoE(30.4m)
Fleeting Expertise, Arcane Energize and Primed Flow for a fair bit of energy economy. You can run Energizing Dash as well (I do) for energy on demand.
Umbral Vitality and Arcane Grace for general defense and healing.
Primed Continuity gives us more uptime on Toxic Lash and better drain economy on Gloom.
Giving Saryn Breach Surge from Wisp is something I've had good results with. The blind will buff your damage even more, give you double procs on the spore damage and help cc the enemies. The radiation procs from Breach Surge will also add another layer of cc, although really not all that important.
Giving her Roar is absolutely an option, you will slaughter crowds faster, but I prefer Breach Surge as it adds some CC to her kit.
Pillage is a compelling option for quick shield gate resets.
Here's the Zaw I use for playing Saryn(It's where my healing comes from!): Link
Once you're done maxing this build, check out my personal Build with Umbral and Aura forma included and the Riven'd Zaw I, personally, think it's Saryn's Final Form!