-This build requires 2 key things to make it work.
The first key aspect is you want to use the Operator Focus Tree, Madurai, and you want to acquire the ability Sling Strength at Rank 3 which gives 40% ability strength for 20s after Void Sling which is basically your operators Dash ability. It seems like in order to activate Sling Strength, you need to do consecutive dashes. You'll also want, Power Transfer Rank 3, for the extra 50% casting speed after entering your Operator form. Which imo, is great for casting [Mag]'s 4th Crush as well as Gloom. Having a slow casting speed can kill a Tenno

-The 2nd key aspect is you want the arcane, Molt Efficiency. Molt Efficiency is an Arcane acquired from the NPC vendor, Cavalero, located in Zariman.
You'll need to be Guardian Rank in The Holdfasts syndicate to purchase this Arcane for 7,500 standing. It also drops from Viod Angels= 2.5% drop chance, Ravenous Void Angels=1% drop chance, Thrax Centurions=0.5%, and Thrax Legatus=0.5%.
However the drop change is quite low.

-The 2nd arcane is self-explanatory. Arcane Energize : 60% chance to replenish 150 energy on self and allies within 15m on Energy Pickup. With a 15s cooldown.
[Mag]'s a caster frame, she needs energy and gloom can be demanding but luckily we have 125% Efficiency with 80% Duration as well as the Added 36% potential duration from Molt Efficiency if her shields are active. The point of the build is to keep Gloom up constantly to slow the enemies, adding an extra layer of survivability as well as health gain to Mag, and thus giving you a constant 36% ability Duration buff in the process.

-Weapons:
So the process of maximizing the effectiveness of everything you have to work with is, first and as always with [Mag], you want to mainly rely on your weapon to do most of the work. A good weapon means you kill things faster, thus surviving longer. So I'd recommend either a weapon with good Punch-Through or a weapon with good AoE. Magg's only true damage ability in later levels is her 2nd, which is Magnetize. Magnetize can shredd health bars in seconds and can out DPS a lot of frames in multiple scenarios but in order to do this you'll need a good weapon. Personally, I recommend Tenet Arca Plasmor as your primary and Kuva Nukor as your 2ndary. The reason why I recommend Kuva Nukor is because of Nullifiers. Nullifiers can be [Mag]'s worst enemy, however, her Polarize (Which we got rid of for Gloom) is actually pretty good against Nullifier bubbles, yet higher levels it's not as potent and since we're getting rid of Polarize for Gloom, we need a weapon with a decent amount of firerate and multishot. As for the Primary, Tenet Arca Plasmor has infinite punch-through and shooting into Magnetize will obliterate enemies in seconds.

-How you want to play this build:
So aside from the weapons, first you'll want to use Void Sling to give yourself that extra 40% ability strength for 20secs. After proccing Void Sling for that extra 40% ability strength bring your strength to 269, you'll want to cast your Gloom, Pull enemies towards you with your 1, cast your 4th Crush to give you and your allies some overshields and repeat the Process of Pulling and using Crush. Use Magnetize when you start getting overwhelmed, preferably in doorways while in bigger rooms or in smaller corridors. [Mag]'s Pull has an insane synergy with a high strength Gloom build. Since it's cheap to cast and knocks them to the ground, they'll spend a very long time getting up.
(Also note that just going into Operator form gives you 50% casting speed 20secs)

-Also remember, big open rooms can be bad for [Mag] since enemies tend to have insane aimbot at long distances. (You don't want anything outside of your Gloom radius to see you which will be 30m at max range). Luckily if you find yourself in a nasty situation, you have 3 different choices to choose from. A: cast your Pull to knock them down, B: cast your Magnetize on the source, or C: cast your Crush to knock everything around you to the ground while also giving yourself some shields. I honestly prefer to just Pull then cast Crush since Pull is a really fast Cast and cursh gives shields. I tend to use Magnetize as my trump card.

-Magnetize can do wonders with how your game turns out as it provides very useful CC, Damage, while also providing cover if need be since Magnetize also sucks in enemy projectiles. Also, with the new Exmius buff, your top priority targets will always be Eximus units so Magnetize will be your best friend against these guys since their Overguard shields aren't immune to Magnetize and any projectiles they fire will just be sucked back in dealing damage to themselves. Magnetize may not stop them from moving but it will make them an easy target while also protecting you from their attacks. Just remember since Magnetize sucks in enemy projectiles, it can also kill you if you get too close to the middle so stay on the outer regions of Magnetize

Side Note: If you don't have Primed Sure Footed, I'd suggest using either Sure Footed, Handspring, Cunning Drift, or Enemy Sense. In that order. Since getting knocked down can ultimately lead to your death, you want to spend as much time off the ground as possible. It's never a bad thing to have too much range when you consider Gloom and Crush, so Cunning Drift isn't a bad choice. Enemy Sense can also keep you alive since it'll show you where they're mostly coming from.

This builds also Steel Path Viable