[Xaku] is deceptively good. I've really been enjoying playing him, so I thought I'd post the best build I've found for others to enjoy.
I've found the best ability to helminth on is [Harrow]'s condemn to use shield gating for extra survivability.

The Playstyle

With this build, you primarily let stolen weapons do the work. At mission start, get some energy, either through starting with it with yellow archon shards or using an energy restore pickup or using Zenurik. From there, once you encounter more than one or two enemies, use Grasp of [Lohk] to steal their weapons, then the Vast Untime to pause their duration. Once you get to a location that you're going to be spending some time at, use your 3, with the Gaze sub-ability, to freeze an enemy and create a giant area of stripped armor. When Vast Untime runs out, recast it to keep your weapons up and the armor strip area up. Recast Grasp of [Lohk] at some point to max out the number of weapons you have. When you get hit and your shields run out, cast Condemn to restore them above full nearly instantly. With 100% armor strip from [Gaze], 16 weapons from Grasp of [Lohk] will kill everything including high level steel path enemies very quickly, and if you properly use condemn with shield gating you're effectively immortal (Just watch out for toxin procs and venemous eximus).

If you're going for a long time, let Grasp of [Lohk] time out and steal new weapons, as the weapon damage scales with enemy level.

In Depth Build Details

Helminth Ability
This build relies on weapons stolen by Grasp of [Lohk] to deal damage. [Xaku]'s first ability, [Xata]'s Whisper, doesn't benefit his abilities, and as such is the ability to override. I use [Harrow]'s Condemn to restore shields to use shield gating to stay alive with ease, but if you're looking for a more offensive option [Wisp]'s breach surge adds a lot of damage. If you're looking for alternate defensive options, [Banshee]'s Silence works well. Gloom also does well, but as we run negative efficiency and maximum range, it can be hard to sustain.

Mods
The most important stat on [Xaku] by far is ability range. Ability range affects both the number of weapons he can have stolen at once, AND how close enemies have to be for the stolen guns to shoot at them. In addition, having extremely high range on [Gaze] is very nice, as a bigger area armor stripped is very convenient. As such, we cram every ounce of ability range possible onto him.

Ability strength and duration are both important on [Xaku]. You need to have 200% strength total on him for [Gaze] to fully armor strip, and the more power strength he has the more damage his stolen weapons do. Duration is very important to keeping your stolen weapons and armor strip zone up, but with the Vast Untime's ability to pause the durations of his other abilities, as long as you have decent energy income you can make the last virtually forever.

[Streamline] and [Flow] are there to help overcome the hefty efficiency penalty from [Blind Rage], as well as the generally energy hungry nature of [Xaku] in general.

[Xaku]'s base armor and health are very low, so increasing them won't do you much good. Better to rely on shield gating than trying to tank hits.

Aura
This is largely personal preference. [Corrosive Projection] lets you hit full armor strip without needing to get all the way to 200% power strength, but it won't be long with molt augmented to get to that point anyways. I personally think [Energy Siphon] is more useful, but no one choice is far and away above the rest.

Arcanes
Arcane Energize is very very nice. It essentially makes it so you only need one or two energy pickups to be able to recast Vast Untime. Not essential, but if you don't have it you should probably run with [Transient Fortitude] instead of [Blind Rage]. Molt Augmented makes scaling our ability strength up very easy. It's how we hit the 200% ability strength breakpoint where [Gaze] fully armor strips. You can instead run [Power Drift] or some power strength archon shards to make up the difference.
[Xaku] might have 75% dodge chance, but in high level content, that 1 in 4 projectile hitting you will just kill you. As such, you really don't benefit from tank arcanes like Arcane Grace and Arcange [Guardian].

Archon Shards
Archon shards give the build a lot of flexibility. The three best:
1. Ability Duration (Red)
Massive quality of life enhancement.
2. Ability Strength (Red)
Extra damage, and can help get you to 200% power strength for [Gaze] to fully armor strip faster.
3. Starting Energy (Yellow)
Very nice quality of life enhancement. [Xaku] needs quite a bit of energy to get started in a mission, and this is an easy way to get it.

Alternatives
If you feel like dropping an aura forma on, you can use [Brief Respite] for an aura and subsume on an ability other than Condemn.
If you don't mind recasting Vast Untime more often, running [Fleeting Expertise] instead of [Streamline] or [Flow] will really help with energy sustain.

Well, I hope someone finds this of use and enjoys [Xaku] as much as I do! If you have any questions feel free to ask!