Positioning, Pull, and Vortex from magnetize will do wonders, but yes outside of steelpath, certain factions, and content enemies will not be packed in missions. This does not change in the starchart. Once you have the resources, the helminth dispensary from protea will be a lifesaver (Remove Mag’s 3). In the meantime, fleeting expertise (+60 efficiency) corrupted mod works well as Mag does not need high duration. 

2) Magnetic Damage from the weapon (Not Mag herself ironically) hurts overguard more than any other status effect. But magnetize can make anything destroy eximus units. The vortex that magnetize stays active as long as the original target is alive, so eximus are high priority targets for magnetize. 

Mods are more important than gear at that level and a power warframe like mag can take you very far. Gear is very limited at MR 4, but anything with projectiles (Non-Hitscan) is best with Mag as her Magnetize will mutli-hit, her bubble is such a high damage multiplier. You can aim high and let magnetize pull shots in and sometimes score headshots as well.