You should train a Timid Magnezone,and i'd say teach it Electric terrain,because field effects can save you from a lot of situtaions,If you wanna hit hard with Magnezone which i don't recommend,get a Modest one.
Volt Switch/Discharge
Flash Cannon
Thunder Wave
Electric Terrain
I'd say let it have Sturdy,you can live even if it's Ground type and can set up Electric Terrain,
~
Remember to EV training and you will be fine~
You should obtain Yureyu Key,and level up Magneton in a secret room in Shade's Gym.
Also,
You don't have any Grass type or Ground type,
You should at least have Grass type for utility reasons in your rotations,
And Ground is the most offensive type in the meta-game,which also generally physically defensive and bulky,
A Jolly Mamoswine or Adamant Swampert for the win ~