[2.0] Block Viper Striker A.K.A BlackMamba

Skill Functions and Interactions

A Viper Strike attack does increased physical damage, and then also has a 50% chance per hit of adding a 'debuff' (a degen charge) which does DoT damage equal to 25% of the initial hit (all damage before any chaos bonuses or DoTs are applied). There is no upper limit to the number of debuffs which may be stacked; it is simply a function of attack speed and debuff duration (7 seconds base).

Viper Strike's initial hit is best scaled as physical damage (although elemental scaling is possible as well), with additional modifications from bonuses to chaos and DoT damage.

So Focus to NOD Duration and Damage over time + Block
Basic Duration 7sec if you make more then 7sec recomend 14sec much more damage effect.

Damage Formula

Optimizing damage output involves striking a balance between physical damage, chaos and DoT bonuses. The formula which expresses how these different factors relate to overall VS damage is however not simple.

Since each hit has only a 50% chance to apply a DoT charge (GGG refers to this as a 'debuff,' but visually it appears more like a charge of poison), any consideration of the total damage output needs to take this into account: not every hit will weigh equally. For simplicities' sake, we will simply calculate as if the DoT did half the damage it does - which, on average, is the same as having a 50% chance to apply:
Total Damage = (VS Hit) + 0.5 * (VS DoT)
We'll get better insight into how the total damage depends on physical damage, chaos bonuses, and DoT bonuses if we can express these two basic components of VS damage, (VS Hit) and (VS DoT), in terms of those other factors.

VS Hit

The first of the two components of total VS damage is the VS Hit, or the initial strike. The Viper Strike gem increases the physical damage of the basic attack, and then also applies an additional 10% of this hit as chaos damage. This chaos damage is further modified by any chaos damage bonuses the character may have.

First of all, we need to calculate the physical damage without any global "increased" modifiers. We start with the Weapon Damage (including all physical modifiers found on the weapon - meaning whatever the weapon tooltip shows), and apply any global additional physical damage:
Min = Weapon Min Damage + Added Min Damage Max = Weapon Max Damage + Added Max Damage
The average will suffice for our calculations:
Base Damage = (Min + Max)/2
Now we split off 10% of the weapon damage for the extra Chaos damage, and apply all "increased" and/or "reduced" damage modifiers accordingly:
Hit Damage = Base Damage * (1 + sum(increased Physical/Melee/Attack Damage)) + Base Damage * 0.10 * (1 + sum(increased Physical/Melee/Attack Damage) + sum(increased Chaos Damage))
Next, we apply our Critical Strike chance and Multiplier:
Total Critical Strike Chance = Weapon Critical Strike chance * (1+ sum(increased Critical Strike Chance)) Total Critical Strike Multiplier = 1,5 * (1 + sum(increased Critical Strike Multiplier))) Total Hit Damage = Hit Damage * (Total Critical Strike Chance * (Total Critical Strike Multiplier - 1.0) + 1.0)
Note: I only cover the most basic version of this calculation here. I did not include any added Elemental or Chaos damage, nor any "more/less Damage" modifiers. Since "more/less physical damage" is a common occurrence, to add those you simply calculate the Hit Damage as mentioned above, then multiply it by (1 + more/- less) for each of those modifiers.

Now that we have the Total Hit Damage, the DPS shown in the skill tooltip (which only considers the initial hit) can be calculated simply by multiplying it with your total APS as well as your Chance to Hit (which is quite complicated to calculate - simply check your character screen):
Total APS = Weapon APS * (1 + sum(increased Attack Speed)) Tooltip DPS = Total Hit Damage * Total APS * Chance to Hit
VS DoT

Each tick of each charge does chaos damage of 25% of the initial hit per second, which is then further modified by bonuses to chaos and DoT damage as follows:
Damage per Second = 0.25 * Total Hit Damage * (1 + sum(increased Chaos Damage) + sum(increased Damage over Time)) Total DoT Damage = Damage per Second * 7 * (1 + sum(increased Duration))
This is, assuming the DoT stays on for the full duration.

Duration and Attack Speed

In the formula above, you can see that duration is also a factor for total damage. The default duration is 7 seconds, but this can be increased with either support gems or skill duration passives. More than 7 seconds to kill a mob is however already very slow, most mobs should die much faster than that, and so it's probably only in boss fights where the extra duration could be a benefit. Situational use of an increased duration support gem in those cases would probably be a more efficient tactic than passive node investment.

A complementary benefit can come from attack speed, since there is no upper limit on the number of degen charges that can be stacked.

Vulnerability Curse

Vulnerability is an especially good curse to use with Viper Strike. It has an immediate effect on the initial hit, which directly increases the DoT ticks that follow. Above and beyond that, vulnerability introduces extra bonuses for DoT damage, and so the curse has a kind of double-dip effect for boosting total VS damage.