When Magneton use only electric-type moves, Golem is the best to defeat it. While decreasing damage of electric-type moves, Golem's ground-type moves are super effective against Magneton. Additionally, Golem has higher stats along ground-type Pokemon. Note that when Magneton uses steel-type moves, Golem less work because it is weak to those moves.
Best moveset for Golem to beat Magneton
When fighting against Magneton, players should make Golem learn ground-type moves. Rock-type moves are not very effective against a steel-type Pokemon.
When Magneton use steel-type charge moves, Arcanine is the best one to beat it. Arcanine's fire-type moves are very effective against Magneton while cut damage of steel-type moves. Arcanine has higher stats along fire-type Pokemon as well as Charizard which is weak to Magneton's electric type moves.
Best movest for Arcanine to beat Magneton
Although Arcanine can learn Bite and Bulldoze, those moves are not preferred based on type match-up when fighting against Magneton.
Other fire-type Pokemon also work well. Ember and Fire Blast are recommended for their moves.
If players do not have such fire-type Pokemon, use ground-type Pokemon noted above. They can reduce damage of electric-type moves. Note that Magneton's steel-type moves are super effective against rock-type Pokemon.