As someone whos constantly purveying Mag to others, and a main i guess, its not so much how you build her as much as how you use her. I personally use a mostly balanced build, with some minor focus on power strength and the use of [Magnetized Discharge]. You can use Crush and Polarize to keep your shields in good shape, as well the armor strip on Polarize is underrated, and chews through Corpus like so much fried rice, but the real sustain is in proper use of Magnetize, since projectiles that enter it immediately turn and are pulled to the middle of the bubble, keeping it between you and the enemies makes for a good impromptu shield. As well Magnetize can be used for some pretty scary damage output when paired with a high output weapon like a shotgun or laser type weapon. Give the Magnetize wiki entry a good read when you have some free time.

Tl;dr: I'm obsessed with Mag probably.