Hi, here is my build for [Mag Prime] in Steel Path.

Notes

[Counter Pulse] is really good for Corpus, Grineer, or Corrupted enemies.
But, it is useless for Infested enemy.
That is why Magnetic Discharge +45 range is good in the build.

About [Natural Talent] since we are abusing shield gates with her 3rd ability (Polarize), increased cast speed is needed.
If you think it is not necessary, you can change it with [Streamline].
In steelpath survival gameplay, I rarely run out of Energy with efficiency at 100%.

About Exilus Slot, [Primed Sure Footed] is good option, but I prefer [Power Drift], since it add additional 15% ability strength.
but, if you want it, you need to put 1 more forma though.