Shards:
1x Ability Strength (to full strip),
1-2x Casting Speed (1is fine, but 2 feels better if you cast the Lost more often to move the strip zone around,
1x Starting Energy (Otherwise it's a bit of a hurdle to get everything going, with the negative efficiency and duration)
Rest is flexible

Stats:
Duration: I find negative duration fine, because you want to recast the vast untime often to apply it's debuffs to enemies anyway. But if you aren't a fan, you can swap Transient to [Umbral Intensify] + another strength archon shard.
Range: Enough for 15 guns.
EHP: We are getting armor and health from Arcane Blessing and [Guardian], because there is no mod space, but even if there was, [Xaku]'s base stats are yucky. [Adaptation] is a must slot for ehp tanking that doesn't involve overguard.
Strength: Enough for full strip
Energy: Negative efficiency + Primed Flow + Equilibrium. Fine to go negative, because the 4th pauses durations, so you only spend energy on non-4th's between long periods of time.
If a sick augment for his 1st or 4th releases, [Transient Fortitude] would be the mod to go, would just need to use flex shards on strength.

Focus: Zenurik for the energy economy(my preferred) OR Unairu for 300 armor OR Madurai to to snapshot 40% strength for 1,2.