I've recently read a bit about the Aid action/reaction and I'm now wondering how to apply this as a GM controlling the party's enemies.
The key thing I'm unsure about is:

Critical Success You grant your ally a 2 circumstance bonus to the triggering check. If you’re a master with the check you attempted, the bonus is 3, and if you’re legendary, it’s 4.

(Italic emphasis mine)

How do I figure out easily which proficiency a given NPC or monster has for a particular check? Is there a rule of thumb where it's Expert up to level 13 and then Master from then on out or something? (Trained doesn't do anything different in this specific case so I'm not too concerned about finding a threshold for that.) I guess if I had enough time I could compare the respective stat to where it lies on the creature numbers table and then extrapolate from there to where the proficiencies would lie. For example I take the creature's level and then check if the attack bonus corresponds to an extreme value in which case I use Fighter progression, for a high value I use the regular martial progression, Alchemist for moderate and caster progression for low. But this takes a bit of time while a rule of thumb would probably be easier to apply.