With the 25% damage reduction now offered by shields as well as the shield gating mechanic, [Mag] is actually quite durable. For the most part you only cast 3 and 4 but you can use 2 to nuke a target (if it susceptible).
Abilities
Magnetize: ~3.7x damage for 6 sec and makes it nigh impossible to miss your target, shot the top of the bubble to pull bullets downward for headshots
Polarize: instantly restore ~50% of your shield and bypasses the recharge delay, also strips enemy armor
Crush: CC's enemies, grants shield for each enemy hit by each of the 3 ticks (can grant overshield), updates affected targets during casting animation to include those who enter area of effect afterwards
Mods
[Energy Siphon]: Essentially functions as additionally efficiency in that you don't need to find energy orbs or use Zenurik as often, polarity is compatible with [Corrosive Projection] and [Shield Disruption]
[Redirection] + [Primed Vigor]: Increase total shields, swap [Vigor] for [Augur Accord] if needed
[Adaptation]: Only way to increase damage reduction for shields since armor doesn't apply
[Fleeting Expertise]: It's efficiency...and it has [Mag]'s picture on it!
[Stretch] + [Augur Reach]: Increase range for Crush without losing strength to [Overextended]
[Umbral Intensify] + [Augur Secrets]: Increase shield restored by Polarize and shield generated by Crush
Arcanes
Arcane Resistance: if you are intending to be durable, taking 1 toxin proc and dying is no good and now that we can pass 100% resistance in a single arcane slot, I think it is worth using Resistance especially since slash now does not damage under shields.
Arcane Aegis: 3% chance to restore 30% shields for 12 sec, totaling 360% shield restore. The other option is Arcane Barrier which is 6% chance to restore 100% shields. Barrier will proc twice as often as Aegis and restore 100% each time, effectively restoring 200% shield in the same time frame.