Perfectly capable of carrying in the Steel Path, and surprisingly good at not dying if you're careful

Cast Crush. Enemies will have all their armor removed and you and your allies will have max overshields. Cast ​Magnetize. Enemies will fly into the bubble from a huge range. Cast Breach Surge. Fire into the bubble with an AoE weapon (such as [Acceltra] or [Tenet Envoy]) or a punch through weapon (such as [Tenet Arca Plasmor] or [Stropha]). Everything dies. Stand in the bubble to avoid getting shot. Cast Pull to bring more enemies into the bubble. Bubbles can be popped early or left for their full duration as needed.

My weapons of choice to use with this build are:

• This Tenet Arca Plasmor build is extremely good because the projectile is huge, does massive damage, and hits many, many times before it expires
• This headshot Acceltra build is very good because you can just fire into the top half of the bubble for easy headshots
• This headshot Trumna build also gets easy headshots, and firing the grenade into the bubble makes it explode several times per second similar to the Tenet Arca Plasmor
• This Stropha build acts similarly to the Tenet Arca Plasmor. Fire into the bubble 2-3 times to max out the combo multiplier and then make a heavy attack or two
• This Kuva Nukor build is not as good as it used to be since it was nerfed, but it's still decent at quickly chewing through everything in the bubble

Any of these will melt enemies on the Steel Path, and the [Tenet Arca Plasmor] and [Stropha] in particular can kill high level sentients fairly easily. [Kuva Bramma] and [Tenet Envoy] also work well, but they're a little cumbersome to work with and don't perform as quite well as the weapons listed above, so I don't use them regularly with Mag

Ability overview, ranked by importance/usefulness

• ​Magnetize (2) creates a bubble centered on the targeted enemy, and pulls nearby enemies towards the center. All bullets that hit the bubble (even hitscan ones!) are redirected towards the center. 25% of the redirected damage is absorbed and dealt twice per second to enemies in the bubble. If the primary target dies, then when the bubble pops, it explodes and deals high damage in a huge radius. Explosion damage is based on damage dealt to enemies in the bubble after the target died. The explosion is great at killing enemies outside bubbles and even better at opening containers three tiles away. Its augment, [Magnetized Discharge], increases the range, has a chance (that scales with ability strength) to disarm enemies hit with the explosion, and lets you pop the bubble early by recasting while looking at it. It's a very good augment

• Crush (4) stuns and deals flat damage to enemies within a huge range (and through walls!). It also grants a small amount of shields/overshields to you and allies within range for each enemy in range, which can easily max out overshields in 1–2 casts. Decent CC, amazing shield buff. Its augment, [Fracturing Crush], strips 75% of armor (base, affected by ability strength), meaning it can completely remove armor in 1–2 casts, and it immobilizes affected enemies for a short duration. It also increases the cast speed of Crush by 50%. It's an extremely good augment

• Pull (1) deals a flat amount of damage and knocks over enemies in a huge cone (also through walls), pulling them towards you and clustering them in front of [Mag]. Decent short duration CC, great grouping ability. Very useful for pulling large groups of distant enemies into your bubbles or into easy melee range. It's also good at stopping enemies capturing points in interception since it can [Reach] across the entire map. Its augment, Greedy Pull, pulls pickups towards you, which is neat but not really worth a mod slot

• Polarize (3) does a flat amount of damage and armor/shield strip, and it restores a flat amount of shields to you and allies. Stripped armor creates projectile shards that can be picked up by Magnetize to deal more damage, or that can be picked up by Mag to orbit her and deal damage on contact with enemies. It doesn't scale well, and the damage and armor/shield strip are marginal in higher level content. Its augment, Counter Pulse, jams enemy weapons and robotics for a very short duration, which is nice, but you have other, better, CC options

• Passive automatically pulls pickups within 8 m towards [Mag]. A slightly shorter range version of [Vacuum] or [Fetch], but unfortunately does not stack with either mod

Helminth abilities

Polarize is a good ability to replace with the Helminth because its damage, armor/shield strip, and shield restore are all small, flat amounts that don't scale well; It will only kill very low level enemies, and Crush restores far more shields. Alternatively, Pull is arguably better to replace since it has less damage and no armor/shield strip, but I like it and I find it really nice for vacuuming everything into your zone of death. Go with what you like!

There are a few good abilities to put on:

• Breach Surge ([Wisp]): Bolts of energy stick to enemies within a radius and line of sight. Affected enemies are stunned, blinded, and when damaged have a chance (guaranteed when killed) to spawn a bolt that chains damage to another enemy (with a multiplier if that enemy is also affected by Breach Surge). Bolts can apply radiation status. This is absolutely fantastic when paired with Magnetize; the constant damage ticks from the bubble mean that enemies continue spawning bolts until they die, and the damage from the bolts is added to the bubble damage. It's a fantastic boost to damage – not that you need it – and it can just kill enemies if there are enough of them grouped together, even at extremely high levels. Beyond that, it's a great CC ability with much more duration than Crush or Pull

• Spectrorage (Garra): Spawn a wheel of mirrors that distracts enemies and damages them when they attack their reflections. Mostly useful because of its augment, [Spectrosiphon], which gives enemies that die inside the wheel a 50% chance of dropping an energy orb. Useful if you're having trouble with energy generation; just drop a wheel around your bubble

• Pillage (Hildryn): Does a percentage of armor/shield strip rather than a flat amount like Polarize. Radial, but requires line of sight. Useful if you don't want to spend a mod slot on Fracturing Crush, though it only removes 1/3 as much armor, so you'll need more ability strength or multiple castings to fully remove armor. It doesn't grant shields to allies, but it does remove status effects from them (and you). Also restores your shields, but they should be maxed most of the time from Crush anyway

• Tharros Strike (Styanax): Also does a percentage of armor/shield strip, this time 2/3 as strong as [Fracturing Crush]. Heals yourself a bit for each enemy hit as well. Note that this only affects enemies in a cone in front of you, unlike Pillage or [Fracturing Crush], which are radial

Now for the specifics of the build

Stats and core mods

More range means Magnetize will pull in more enemies from further away. It's your most important stat, and it should be as high as you can make it go

More ability strength means higher damage on Magnetize and Breach Surge, enemies get pulled into the bubble harder by Magnetize, more shields restored per casting of Crush, 100% armor strip in a single cast on [Fracturing Crush], and higher disarm chance on [Magnetized Discharge]. The 139% baseline shown here is acceptable, but ~200% is a good amount to have if you can get it (the 139% here assumes you're using Molt Augmented and/or Crimson Archon Shards), and more is better

Duration is nice but not that important. 100% or even a little less is fine

Efficiency can be dumped if you have a large reserve of energy from (Primed/Archon) Flow and/or Azure Archon Shards and a way to generate lots of energy, like Zenurik's Wellspring, Arcane Energize, and/or Amber Archon Shards

Other Mods

[Magnetized Discharge] and [Fracturing Crush] are both fantastic augments, and I consider them mandatory

[Adaptation] is great since it works on shields, and you have a lot of shields that can be recovered easily

If you use the helminth to add an ability that does cold damage, consider replacing [Stretch] with [Archon Stretch]. If you add an ability that does electricity damage, consider replacing [Primed Flow] with [Archon Flow]

Use whatever aura mod you like best. I like [Rejuvenation] because it helps with survivability. [Brief Respite] is nice, but unnecessary with Crush (and Polarize or Pillage)

Arcanes

Arcane Energize is enough to be able to spam abilities, even with reduced efficiency. If you don't have it, consider trading some ability strength for efficiency, using Zenurik's Wellspring, and/or increasing your energy per orb with Amber Archon Shards

Molt Augmented brings ability strength to 199%, but takes some time to spool up. If you don't have it, or if you want more immediate ability strength, consider replacing [Augur Reach] with Transient Fortitude. Crimson Archon Shards can also help bring ability strength up to par

If you address energy consumption and/or ability strength in other ways, then use whatever arcanes you prefer. There's lots of good choices!

Archon Shards

Finally, use Archon Shards to taste. Personally I like to crank up ability strength, but there are lots of good choices. Pretty much all of the options can be very good on Mag depending on what weapons / focus school / arcanes you're using and what you prefer. Go all in on one buff to push it as far as possible or mix and match to get several quality of life improvements