Easy. Simply get a magmar (get in the safari zone or burned tower) and give it a magmarizer (on cinnabar island) then trade it.
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No. Its impossible to get a Magnezone without action replay because it can be only be obtainable in pokemon Pearl/Diamond/Platnium. Sorry.
To get a magnezone you have to level your magneton up on mount coronet
Magnezone is a Generation 4 (Diamond, Pearl, Platinium onwards) Pokemon so cannot be found/obtained in Fire Red.
There is no best Pokemon on Pokemon HeartGold.
there is no Pokemon Mansion in Heartgold.