My personal build, using an umbral and aura forma.
Power Strength: [Blind Rage], [Umbral Intensify] and [Augur Secrets]: Stacks Spores quicker and give Gloom a higher Slow %
Power Efficiency: [Primed Flow], Arcane Energize and Energizing Dash lets us make the most out of the orb drops we get and helps mitigate the drain from Gloom.
Power Range: [Stretch] and [Overextended]: Give Spores and Gloom enough range to keep enemies stacking/cc'd
Power Duration: [Primed Continuity] lowers the drain of Gloom, lowers the decay rate of spores, duration of Miasma's sickness and uptime on Toxic Lash
Defense: [Umbral Vitality] for a big pool of health.
Forma'ing the exilus slot to a D polarity for [Primed Sure Footed] is a compelling option cause I just [Hate] getting knocked down, dragged, staggered, etc. Yea I know you can melee block and not be affected, but I kinda like the brainless approach of slotting a mod and not having to worry about it.
This is the build I use for Steel Path, we could put [Umbral Fiber], [Adaptation], Arcane Grace and Arcane [Guardian] on and we would still die extremely fast. I feel Gloom lowers the attack speed of enemies enough to stay alive DECENTLY but running Pillage instead of Molt/Gloom gives us a panic "heal" (Refilling shields, overshields and resetting the shield gate). This has helped me big time with staying alive long enough to kick out [Saryn]'s dps.