By default, adversaries don't normally make action rolls except for attack rolls and any unique actions described in their stat blocks. Any other action an adversary attempts simply succeeds without an action roll; if you want an adversary's action to have a chance of failure, have any relevant PCs make reaction rolls instead.
However, for especially dramatic or dicult tasks that the PCs can't influence, you can give an adversary an action roll anyway. For an adversary's action roll, roll a d20. If the result is equal to or greater than the action's Difficulty, the action succeeds—otherwise it fails. You can spend a Fear before rolling to add a relevant Experience from the adversary's stat block to the total. Use the same procedure when an adversary makes a reaction roll.