Neither Thunderbolt nor Charge Beam fit well into your Dual Screener Magnezone Set. The best Electric Type Move for that Set is Volt Switch, as it provides Magnezone with a decent STAB that switches into the Pokemon that Reflect and Light Screen is ment to protect safely due to his Low Speed.
I also suggest using Hidden Power [Ice] in place of Flash Cannon if possible, as Electronic+Ice scores excellent Coverage that hits every Pokemon for Neutral or Super Effective Damage (bar Rotom-H and opposing Magnezone).
Here is the suggested Set:
Magnezone @ Leftovers
Trait: Magnet Pull
EVs: 200 HP / 52 Def / 4 SAtk / 252 SDef
Relaxed Nature (+Def, -Spd)
0 Spd IVs
- Volt Switch
- Hidden Power [Ice] / Flash Cannon
- Light Screen