My Personal Build

The focus of this build is high mobility, high versatility, and spreading as much status as possible for as long as possible.
Note: This is a VERY intensive build that requires you to manage multiple cooldowns, not for the faint of heart, but great for people who want to feel like they're doing something every moment

Arcanes:
Arcane Impetus can give us +60% Strength and +30% Efficiency through uses all of [Lavos]' infususions, once at max stack, you only have to casue a status effect on another enemy with any ability and it will refresh the timer, some companion abilities also count for this

Arcane [Ice Storm] can give us +40% Strength and +40% duration upon enemies frozen. this has a tighter time limit of 15s, but this is why we run a lot of range, to utilise vial [Rush] which can quickly freeze a few enemies

Both of these arcanes together can get us to:
Duration: 195%
Efficiency: 130%
Range: 205%
Strength: 140% (230% on Catalyze)

Range and duration are our most important stats with this build, allowing for longer status times with all our abilities and huge mobility with vial [Rush]
By using both [Swift Bite] and the transmutation probe, you should be able to use catalyse quite often

If you find you don't need valence fusion, then you can swap it out for another strength mod like [Amar's Hatred], [Augur Secrets], or even [Blind Rage] if you're happy to take the hit to your cooldown reduction abilities

Shard Options

Emerald Shards for Corrosive stacks or Toxin Heal
Topaz Shards for Casting Speed
Crimson Shards for Ability Duration and Strength

I'm running:
1x Emerald Shard Corrosion Stacks
1x Emerald Shard Toxin Heal
1x Topaz Shard Casting Speed
2x Crimson Shard Ability Duration (Bringing Duration to 225% with max Arcane [Ice Storm])

My Loadout:

Primary:

Infinite Ammo status AX-52
This is a great way of proccing a huge amount of statuses while being able to fire constantly, great for getting shots in at the mecha target too

Alternate choices I like are the classic [Cedo Prime] or a Super-crit focusesd Gotva Prime which can fully take advtange of [Valence Formation]

Secondary

I'm using a conjuction voltage Bronco Prime since you can utilise [Valence Formation] for electricity for huge amounts of status effects + slash procs which can spread via the mecha set

Melee:

An influence Tenet Grigori is my go-to here, but realistically, any melee influence weapon is amazing here, if you build it for status damage without electricity, you can once again use [Valence Formation] in order to force an electric proc on it giving huge amounts of damage output

My alternate choice is a Melee Vortex Dual Viciss which spreads gas and groups enemies which benefits Mecha set a LOT, this also doesn't rely on an infusion, so is a great backup weapon for quick swings

Companion

Toxin Support Pharoah Predasite is my goto, but any kubrow does the job for the mecha set
Note: If your companion has [Seismic Bond] slotted and deals damage while you're using vial [Rush] (which should be often in this build) then you can get extra efficiency bringing is up to 160% which benefits ophidian [Bite] and transmutation probe a lot