Part of the Lucky Devil achievement is to unrust a Slighty Rusted armor to get a Golden Armor. Only the
Slighty Rusted Armor will give you a chance to get a Golden Armor, the other rusty armors will not. To get the Polisher Achievement you need to successfully unrust 1000 Rusty Armors it doesn't matter the type. I'd suggest doing these two achievements hand in hand if you want to give yourself motivation to loot the armors!
According to Tibia Fandom it can take 1061 tries to get one Golden Armor from unrusting JUST slightly rusty armors. So in order to get the Lucky Devil and Polisher achievement I'd suggest focusing on the monsters that drops just the slighty rusty armors, buying is way easier but if you want to loot them I'm going to recommend two ways based on your level just based on experience and the drop chance.
If your a high level I'd recommend Ghastly Dragons to loot the slighty rusted armor it's considered uncommon (drop chance between 5-25%) These take forever to kill compared to the others but they frequently drop slightly rusty armors compared to the others so I'd suggest Ghastly Dragons if your a higher level.
However if your a lower level I'd suggest
Metal Gargoyles or
Rustheap Golems in Oramond. To loot the slighty rusted armor it's considered semi rare (drop chance between 1 and 5%)
Source: https://tibia.fandom.com/wiki/Slightly_Rusted_Armor