OG Assassin

Note: Not to be used with non-crit incarnon weapons

Archon Shards
5 Crimson equals +50% to 75% duration
[Narrow Minded] alone gives 15s of invis
With [Narrow Minded] and Molt Efficiency, you get 18s
With [Narrow Minded], [Primed Continuity], and Molt Efficiency, you get 23s
With Narrow Minded, Primed Continuity, Molt Efficiency, and 5 Crimson Archon Shards, you can get 27-29s

Helminth
[Xata]'s, Roar, Eclipse

Warfame Arcanes
Molt Efficiency for extra duration, Molt Augmented for additional 60% strength over time
Arcane [Rage], Arcane Rise, Arcane Primary Charger, Arcane Acceleration, Arcane Tempo (for shotguns) for Primary Weapon Damage
Arcane Awakening, Arcane Precision, Arcane Pistoleer, Arcane Velocity for Secondary Weapon-focused builds

Operator Arcanes
Magus Elevate for healing Warframe if not using Vazarin
Magus Lockdown

Operator Focus
Zenurik for energy management, Vazarin for damage immunity using Protective Dash

Aura
[Holster Amp], [Shotgun Amp], [Pistol Amp], [Rifle Amp] for addtional weapon damage
[Corrosive Projection], [Enemy Radar]

Exilus
[Primed Sure Footed] or Mobility Mods such as [Mobilize] and [Amar's Anguish], or [Preparation] (if not using Zenurik Focus School)

Flex Slots
Replace [Vigorous Swap] or [Seeking Shuriken] with [Constitution] or [Augur Message] for more duration, [Energy Nexus] or [Equilibrium] for energy [Regen]
Replace [Primed Continuity] with [Precision Intensify] for strength increase to 289% for the helminth damage ability

Gameplay
Cast (Hardened) Wellspring at start of mission
Cast Smoke Screen, make sure to stay invisible as much as possible
Use your weapon and helminth ability to deal damage
Switch weapons constantly to increase damage
Parkour away from AOE status effects
Heal self through operator transference / Magus Elevate / Vazarin Protective Dash
Use [Rolling Guard] before invisibility runs out
Recast (Hardened) Wellspring to bring energy up to acceptable levels

Alternative Builds

Stealth Operative for Quick Missions
https://overframe.gg/build/608728/ash/stealth-operative-for-quick-missions/

Savage Silence Ash
https://overframe.gg/build/542970/