Greetings!

This build easily makes [Gara] one of the tankiest frames in the game through pure EHP, and this also buffs the crap out of [Gara]'s 4th ability, so buckle up.

warning: this is an extremely expensive build and probably not worth it unless if you love Gara or are looking for a good defense objective warframe

TL;DR

With all buffs active:

• 2242 Armor for Mass Vitrify
• ~1,118,000 EHP
• ~38k Mass Vitrify segment health
• Replace Shattered Lash (1) or Spectrorage (3) with Elemental Ward
• Set Emissive colour to be Cold
• Would recommend you bring extremely good weapon loudouts since you won't have any armor stripping (eg. secondary weapon primer w/ heat & corrosive for a good condition overload melee)

Mod selection notes:

• Adaptation
  • Gara is so incredibly tanky with this build that you can choose to not have Adaptation depending on how long you plan on staying in your mission
  • Replace with Mending Splinters; it can heal both you and the Defense Objective (on top of the ability giving it 90% DR)
  • Replace with Streamline?
• Primed Flow
  • Optional, but recommended because you're bound to have energy issues if you're playing solo
• Primed Sure Footed
  • Most people don't have this; replace with [Handspring]

Have fun!

Helminth Subsume

You're looking for a subsume that'll increase your armor, both to be extra tanky, and for Mass Vitrify's health per-segment.

Options:

Elemental Ward (recommended)

• Consistent armor buff
• Reflects incoming projectiles

Parasitic Armor

• Needs Redirection somewhere in the build
• Gives more armor than Elemental Ward would at 276% strength
• You won't always have the full shields to make this worth it

The winner here, to me, is Elemental Ward. In higher level missions, your shields will usually never be full enough to use Parasitic Armor, so the consistency from Elemental Ward is much better.

Armor Breakdown

I've built for armor since it adds Mass Vitrify's health-per-segment and can give insane damage reduction. Overframe doesn't calculate segment health with armor in mind, only Ability Strength. It also doesn't calculate Armor and Damage Reduction with Elemental Ward.

How much armor are we getting?

• Gara Prime has 190 base armor

Our modifiers are:

• Umbral Fiber (1.1) + Set Bonus (0.825) = 1.925x
• Arcane Guardian = 900+
• Ability Strength = 1.76
  By the end with Cold Elemental Ward, we get:

Total Armor = Base Armor x (1 + Armor Mods + Ability Modifier x (1 + Strength Mods)) + Arcane Guardian
= 190 * (1 + 1.925 + 1.5 * (1 + 1.76))
= 190 * 7.065
= 1342 + 900 (Arcane Guardian)
= 2242 Armor

My god..

What does this mean?

Firstly, Armor gives damage reduction to your health. Anything around 90% damage reduction will almost make you unkillable. The fact that we're also stacking 2 other damage reduction buffs makes you essentially unkillable.

Damage Reduction (DR)

Damage Reduction = Net Armor / (Net Armor + 300)

= 2242 / (2242 + 300)
= 0.88 
= 88% Damage Reduction

EHP Multiplier: 9.61

"Ok, but show me big number go up"

Effective Health (EHP)

Effective Health, or EHP, is the health you'd approximately have with DR from several sources like armor, abilities, and passives.

In our case, we have THREE sources of DR:

• Armor + Elemental Ward + Arcane Guardian (88%)
• Splinter Storm (90%)
• Adaptation (0-90%)

EHP = Nominal Health / (1 - Damage Reduction)

= 1284 / (1 - 0.88)
= 10,700 EHP

Ability and [Adaptation] DR applies to shields too, unlike health, so now we're using the total of 1284 Health + 300 shield.

90% DR has an EHP multiplier of 10.0x, so with Splinter Storm and our shields we get:

10,700 + Shields (300) = 11,180
= 11,180 * Splinter Storm (10) = 111,800
= 111,800 EHP

With fully stacked Adaptation:

111,800 * 10
= 1,118,000 EHP

I hope the math here is right, but from my testing, I can confirm the tankiness is likely this absurd.

1,118,000 EHP. What now?

Armor is also used in the calculation for Mass Vitrify's per-segment health. With this setup, this is how much you'd get:

Segment Health = (Base Health + (5 × Total Armor)) × (1 + Strength Mods)
= (2550 + (5 * 2242)) * (1 + 1.76)
= 37,977

So if you have Arcane [Guardian] up, you'll get around 37,977 health per segment!