With this build you maximize the use of all of their abilities.

[Xata]'s Whisper gets a bonus 52% Void damage which is further enhanced by keeping The Vast Untime up. Add on Grasp of Lohk in crowded areas/choke points, and they too get bonus damage from The Vast Untime.

200% Power Strength is the soft cap needed for [Gaze] from The Lost to strip 100% defenses. [Xaku] isn't necessarily a "press 4 to win" nuke frame, but the amount of damage capability they can accomplish is amazing.

You can save a Forma by using [Augur Message] instead of [Constitution]. I just like having that couple more percent of Duration and the Knockdown recovery.

My only complaints are the energy costs and cast speeds of their abilities. [Xata]'s Whisper should be just as fast as [Saryn]'s Toxic Lash. All of the other abilities should also have their cast speeds cut in at least half. The 100 energy cost, energy drain, and duration on The Vast Untime is completely unfair for upkeep. I would suggest either removing the duration and reduce the casting cost to 25, or remove the energy drain and increase the duration from a base 25 seconds to a base 30 seconds. Until then, this is why we stack [Energy Siphon], [Primed Flow], and Arcane Energize (whilst also running Zenurik) to maintain [Xaku]'s energy as best as possible. Hopefully there will be some hotfixes to bring [Xaku] into a bit easier to manage state.

Let's assume DE makes these changes to [Xaku]. Then I would suggest swapping [Primed Flow] for [Augur Message] for more duration, or [Natural Talent] to fix the casting speed of their abilities. And if you really like Xaku, toss in either a Forma or Aura Forma to use Steel Charge and/or have some flexibility with the Aura Slot for a total of 7 forma build yikes!

Imho, if DE fixes the energy costs and casting speeds of Xaku's abilities, they would be at least an A-tier, if not bottom S-tier Warframe.