Intro -
This build is for use alongside any frame using subsumed Nourish ability, which will add Viral damage to the weapon. If you are not using Nourish, see the Flex Slots section of this guide to make changes to the mods.

Heavy attack only when Tennokai prompts you to retain your combo permanently up to 12×. Any tennokai mod is fine, so if you have the others, use what you like.
I use this exact build with my Styanax, and can deal >100k critical light attacks and anywhere from 700k-4millon damage tennokai attacks during Netracell missions (Level 220-240 enemies) using this exact build with Nourish. No arcane, no special tennokai mod.

Flex Slots
1. Elements
[Shocking Touch] is used here to give Corrosive damage, under the assumption that you are running Nourish to get your Viral damage. If you are not using Nourish, remove [Shocking Touch] in favor of North Wind. Alternatively you can use any other combination of elements depending what you're up against.
Why not Dual-Stat status mods? This build is mostly focused on crit heavy attacks, but a lot of its damage scaling comes from Condition Overload, which means that dual-stat mods should be better, right? Well, not necessarily. Dual-stat mods will give you more consistent status procs and therefor more consistent damage values. Flat elemental mods will cause your damage values to be a bit more unpredictable, but the higher end hits will be way more powerful than those of the dual stat mods. Consider your choice of flat element or dual element based on your Focus School and choice of your next Flex mod below.

2. [Drifting Contact]
   This is a potential Flex slot as well if you are using the Naramon Focus school. If you are NOT using Naramons Focus, [Drifting Contact] should remain on.
   If you are using Naramon Focus, then you don't need [Drifting Contact]. Remove it in favor of Weeping Wounds to get all that status chance we talked about above. If you do this, there is absolutely no need for the dual-stat elemental mods. If for some reason you don't have [Weeping Wounds], [Melee Prowess] can give you a decent amount of status chance for little investment. Options for extra survivability are mods such as [Focused Defense] for blocking (which will also increase the effectiveness of this weapons unique passive), Healing Return or Life Strike for healing. Additional options could be (Primed) Reach or [Spring-Loaded Blade] if you really like the range, or any Faction damage mods.

3. [Berserker Fury]
   This can be swapped with (Primed) [Fury], [Quickening], or [Gladiator Vice] for a smaller but more consistent melee speed increase.

Arcanes
Melee Arcanes need more testing, and are not necessary for this build as I mentioned above; with a 12x combo your Tennokai attacks deal absurd damage already.

If you want to use an arcane, I would recommend Melee Duplicate or Melee Animosity for this build.

If you are not using Nourish as mentioned above, change your mods so that the weapon has Viral damage, and then run the Melee Exposure Arcane to get your Corrosive damage.
Alternatively, if you are running a frame with considerably high shields (as I said above, I use it with [Styanax]), use Melee Retaliation Arcane.