| You don't need much equipment versus those guys
The rust monsters will not attack unless you attack them.
As a melee character I use a crossbow against them (bows and arrows are not metal) and switch to my actual weapons when the other monsters are around.
If you are wearing any metal armor (scale, chain, plate) you must remove it (unequip). They don't seem to eat studded leather (at least in my game they didn't) . Same goes for weapons. Fortunately we did not save after my wife lost the Cleric quest sword to the rust monsters. We learned quick!
If you have a barbarian, I really liked barkskin and resist elements here in the Earth area. A real help for those that can't wear armor. | 
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