Table of Contents (use Ctrl + F or "Find in page" to help search for specific sections):

I. Unlisted items that are required/recommended
II. Mod Breakdown
III. The Strategy
IV. Archon Shards
V. Alternative Mod Options
VI. Final Notes

My take on Marked For Death [Trinity], with low Duration and Shieldgate. A wave of her right hand lights the fuse; a cast from her left hand detonates the field. This is the Master Spellcaster. For an alternate version that doesn't dump Duration, you can check out my pal Ninjase's build here:
https://overframe.gg/build/243135/trinity-prime/mark-for-death-nuke-trinity/

I. Unlisted items that are required/recommended:

-Viral primer
The nuke will work one of two ways -- either max out the Viral on one target, or spread Viral so that the target and all enemies in the vicinity have 2+ procs each. The damage dealt by Marked For Death will be amplified by the Viral from the initial target as well as by the Viral from anyone else hit by it. Personal recommendation: Epitaph. High SC on quickshots and just over 11.5m radius with Primed Fulmination (can also hold with a glaive weapon for auto-blocking incoming fire, a nice defensive bonus). Other good weapons are beams that chain, such as Kuva Nukor or Gaze, or you could use Bubonico or Cedo if you want a primary Viral spreader.

-One or two back-up weapons
While the Marked For Death ability can take out the majority of enemies in the game without fail, there are some enemies where it falters. Acolytes, bosses, and enemies with innate Damage Resistance (such as Nox) will not be one-shot by the ability, so have something to deal with them accordingly. Personal recommendation: [Glaive Prime] (or [Cerata] for Corpus). A bit stale, but highly effective and can pair nicely with some choice Secondarys. Excellent Primarys of choice: [Phenmor], [Felarx], [Quellor] or [Stahlta]. If you're using a primary Viral primer, you can fall back on [Laetum] for dealing with enemies that MFD cannot. If facing bosses (Assassination, etc.), you may just use a different Warframe unless you're supporting a team with your EV or something.

II. Mod Breakdown:

-[Brief Respite], [Rolling Guard], and [Augur Reach]
First off I'd like to reiterate this is my take for Marked For Death [Trinity], so I recognize there's fair chance other players won't use Shieldgating with her because of her 4; respect to them. My preference is to have as many options as I can fit into an idea comfortably; for me, the ability to Shieldgate off EVERY ability is nice to have. With Brief Respite and 3 Augur mods (Reach and two on the Sentinel/Secondary, depending on your preference), any ability costing 50+ base Energy (which is everything on this build) will give enough to Shieldgate on cast at 105% Efficiency. This way, I don't have to spend so much Energy on Blessing everytime my Shields drop, and I don't have to disrupt play or risk having no Energy to cast MFD and EV since my pool is limited and Blessing takes a big chunk of it. On top of this, Rolling Guard grants status cleanse and brief immunity, which can save in a pinch.

-[Primed Sure Footed]
[Trinity] has Link, and this build has max Range. However it also has minimal Duration. This means Link isn't gonna keep you standing firm for longer than roughly a second. PSF will prevent knockdown for you since you won't have it for yourself. If you make any changes to the build to [Recover] Duration, then by all means use some other Exilus in place. If you don't have PSF, I may recommend the Unairu School as a fallback until you obtain PSF. If you use a weapon like Epitaph for spreading Viral, the knockdown resistance is a MUST.

-[Fleeting Expertise] and [Transient Fortitude]
Most EV builders are no stranger to minimal Duration. While it takes away from the Damage Resistance duration from Blessing and Link, it makes EV pulse quickly and has no effect on MFD. I chose this course of action for one main reason: if you hit an enemy with MFD, but miss it with EV (say another enemy runs in between you two), you'll want to get EV back asap, which means you either wait it out or potentially swap to another weapon and kill the current EV target. Minimal Duration means less wait and no need to swap; if you're okay with killing the incorrect EV target, you can cut Fleeting for [Primed Flow] (to maintain assisting the Energy economy) or something else (by removing Fleeting, it can also open up switching some of the Shieldgate support like Brief Respite, Rolling Guard, and/or your Companion, as your Efficiency will go down which means more Shields on ability cast), but Transient is not as easily replaced.

-[Overextended], [Stretch], [Cunning Drift]
To nuke effectively, you need to hit a fairly broad range. Together with [Augur Reach], you hit 280% Range which is 28m nuking field for MFD. Keep in mind that MFD's explosion does require Line of Sight (only for environment; it can go through bodies without issue).

-[Blind Rage], [Umbral Intensify] and Archon Shards
The total amount of Ability Strength you need from Archon Shards for this build is +45% or higher. If you do not have enough Shards for this, simply swap [Cunning Drift] out for [Umbral Intensify]; +44% will be enough. Simply put, this makes the entire nuke possible, boosting the damage MFD and EV deals when casting. There's a whole equation as to why, which you'll see in Section III.

-Arcanes
At the time of writing this, these slots are entirely flexible. I'm actually trolling with Molt Reconstruct, though technically it works here so that you don't need Blessing, as you'll fully restore your Shields (via [Brief Respite] and Augurs) and your HP (Reconstruct), and the low Duration means you'll not get to enjoy any time for the Damage Reduction anyway. Arcane Barrier is an option to give sporadic Shieldgate assists but is also replaceable with anything you prefer.

III. The Strategy:

With your Viral primer, pile the procs on your target(s), then cast Marked For Death, immediately followed by Energy Vampire on the same target. 238% Ability Strength is the absolute minimum you want to use, based on the math shown below involving a Viral primer that hits with an AoE (this calculation has been tested excessively by yours truly; I welcome skepticism, and I suggest you try for yourself to see the results):

6.25% -- first pulse of unmodded EV, based on enemy HP
238% -- Ability Strength
150% -- Marked For Death damage (this is listed as maxed for the sake of simplicity, as the Ability Strength above will more than hit cap)
225% -- 2 Viral procs on MFD/EV target
200% -- 1 Viral proc on any enemy in range of the MFD/EV target

6.25x2.38x1.5x2.25x2=100% of target's HP** dealt as True Damage in a nuke (rounded down from a decimal)

**IMPORTANT
Be aware that the target should be as close to full HP as possible, as this directly affects the damage dealt by EV's first pulse. If their HP is too low, the nuke is less effective.

Alternatively, you can use a primer to focus on getting Viral x10 on just your MFD/EV target. If you take this course, you'll need to increase the Ability Strength to 251% or higher, which may require almost a full set of Tauforged Shards or swapping out a mod for more Strength (remember earlier I mentioned cutting Cunning Drift for Umbral Intensify?).

6.25x2.51x1.5x4.25=100% (again, rounded down from a decimal)

IV. Archon Shards:

-Ability Strength
As previously stated, you'll need +45% Strength to [Reach] the breakpoint in order for this build to work. That's anywhere from 3 Tauforged Crimson Shards, to 5 normal ones. If it's going to take all 5 slots, I would simply make the exchange for Umbral Intensify, because I want you to have at least one slot available for...

-Casting Speed
Now, this will be based on personal comfort. If you put aside the Crimson Shards for a moment, this stat is a crucial one for [Trinity]. [Natural Talent] exists, which I'll go over later, but if you can settle with a lower amount (such as one Shard for +25%, or +37.5% if you're lucky), you can fulfill the need of this stat easily with one Shard. I recommend getting multiple copies of [Natural Talent] at different Ranks, and go test in the Simulacrum to see if you're comfortable with less than 50%. Personally I felt no difference when I equipped my Tauforged Amber Shard. If you decide you need two of these, then unless you have 3 Tauforged Crimson Shards that you're willing to spend here, then make the Cunning->Umbral exchange.

V. Alternative Mod Options:

Here I'll list some ideas for mods/Arcanes you may consider should you make personal changes to the build (such as cutting Cunning Drift or Fleeting Expertise):

-[Umbral Intensify]
I'm sure I'm sounding like a broken record by now, but this is my largest suggestion as an alternative simply because if you don't have the Shards for the extra Strength, you're not going to have effective nukes.

-Primed Continuity
If you decide you're not keen on minimal Duration, this becomes a viable option.

-[Natural Talent]
This can help increase your casting speed on MFD and EV, since you'll be casting all the time. This will depend on your plans with the Archon Shards, since again 2 of them already match/outperform this mod depending on if they're Tauforged or not.

-[Vampire Leech]
This can assist with Shieldgate if you decide to remove Brief Respite for alternative Aura modding, as excess Energy will be turned into Shields.

-[Abating Link]
Technically an option, but very niche (maybe Acolytes are annoying you too much, or armored Eximus units during long endurance runs). You'll want to run a lot more Duration to make it worthwhile.

VI. Final Notes:

Some key information to remember when using this build:

-When it comes to Eximus units, try to avoid casting MFD on them directly. While they are certainly able to die from the damage from the nuke and EV (unless they have DR, such as Nox), they themselves won't nuke the area via MFD until Overguard is removed. So it's more effective to nuke enemies near the Eximus and let them die with their trash mob. Otherwise, you'll be forced to remove their Overguard to get your MFD back (fortunately, a little Viral and EV can destroy the Overguard in 1-2 casts).

-As I've mentioned before, Marked For Death is a Line of Sight AoE. For tilesets where there's a lot of columns, boxes, barrels, hills, etc. around, be ready for some enemies to be protected from the explosion. This is part of the reason my preference of primer is an AoE like [Epitaph] or [Bubonico]; if they survived the first nuke, they're just going to become the second one.

Overall, whether it's star chart or level cap, this build will prove highly effective across the vast majority of what this game throws at you. Questions or comments, feel free to leave em.