Part of the Lucky Devil achievement is to unrust a Slightly Rusted armor to get a Golden Armor, the other part of the achievement is to fish loot a Leviathans amulet from a Massive/Water Elemental which is more explained by clicking on this link.
Only the Slightly 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 slightly 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. Ghastlys have 7,800 hitpoints.
If hunting Grim Reapers is something you love to do with a team or solo slightly rusted armors are considered semi-rare (drop chance between 1-5%) I typically will leave will on average with 15+ slightly rusted armors for every 400 Grim Reapers. Grim reapers have 3,900 hitpoints.
Plaguesmiths drop slightly rusty armors as well uncommon (drop chance between 5-25%) but I'm not really into hunting them as spawns can be very limiting and annoying in my opinion!
However if your a lower level I'd suggest Metal Gargoyles or Rustheap Golems in Oramond. To loot the slightly rusted armor it's considered semi rare (drop chance between 1 and 5%)
To make your life easier I'd recommend you use prey and and 40% loot to whichever creature you pick :)
Source: https://tibia.fandom.com/wiki/Slightly_Rusted_Armor