Magnezone turned out to be a decent pokemon for me at best late game...and to think I kept a Magneton in my main team for 65 freaking levels!
Of course, if you still want to get it that badly, you need to get the Red, Yellow (?) and Green plugs from Luna to gain access to evolve it. Spoilers ahead...(I know you are not at that part yet but you can look here for future reference)
Go to the passage where Saphira meets you in Tanzan Cave and enter it. Use the green crystal plug on the green crystal and gain further entry. Do the same for the other crystals and you should find yourself in the Byxbysion Tunnels. Once inside, you'll find many rocks to move around, save at this point in case you move them in the wrong place (they were pretty simple imo) and click on each of the three machines. Then a hidden tunnel will appear and go through that. From that point, you must fall in the appropriate holes to gain access to the outside and top of the tunnels where you can find the Yureyu Key. Now, go back to Shade's power plant and unlock the locked rooms where you can find 6 Electrodes attached to a T.V displaying Dr.Connal's sister's death. Level up in that room and your Magneton will evolve into Magnezone.