Just had an idea for a mechanic that could be applied to really tough targets, such as iron animated statues.
If you haven't read the Kill It With Fire rules, let me paraphrase the armor rules here: For each armor die you have, it reduces incoming damage by its pip rating (being removed if matched) and if any damage goes over that, the armor die is removed from play and the extra damage is applied to hit dice (armor dice come back after combat when you repair your armor).
But creatures with the Tough ability will have an extra, armor-like advantage: Each hit die also acts as an armor die, so it first reduces incoming damage by its pip rating then absorbs damage as usual. Meaning some creatures will never lose HP if something can't deal enough damage to beat its highest hit die's pip rating.
There is nothing to stop a Tough creature from wearing separate armor on its body, but this trait is mainly intended for creatures of naturally high fortitude, such as dragons.
Since hit die in KIWF are rolled at the beginning of each combat, you may get lucky and catch a monster with a measly 3 pips on its highest die, or you may run into a particularly strong monster with the dreaded 6 pips. Monsters could be even more dangerous if you are using alternate dice rules as well (KIWF uses d6s in the base rules).