You can likewise use any 5e compatible monster stat block with Nimble with minimal tweaks on the fly. Run them exactly as written, using their move, action, bonus action, etc. as normal; simply change their AC to armor and roll their damage dice for attacks.
- Monster Attacks. They miss, hit, and crit just like heroes.
- Monster Armor. 5e monsters that have 14–17 AC have Medium Armor, less than that is unarmored, more than that is Heavy.
A Note on Armor Encounter balance in Nimble is much more forgiving than other games. A GM can adjust armor up or down quite freely without breaking encounter balance. If too many monsters have the same AC, the GM can upgrade/downgrade their armor for variety. Unarmored should be the bulk of monsters your party encounters (~60%), Medium should be uncommon (only about 30% of the monsters in any given session), while Heavy should be quite rare (~10%).