It is suggested (by the DMG) to account for defensive options by assuming increased AC, e.g. +2 AC for magic resistance.
I would like to give some of my monsters the spell mirror image as part of a spellcasting / innate spellcasting trait. There is precedence for this: the Alhoon, the Faerie Dragon, and the Lamia have it, for example.
What would be a rough estimate of the AC equivalent for mirror image for the purpose of determining CR / combat performance?
Tangentially related (compares mirror image to other spells for PCs concerning the defensive capabilities): What's my most efficient use of spell slots to help my AC?
Best Answer
Frame challenge: use HP
First, monsters with big bonuses to their AC apart from their Dexterity do not benefit much from the spell, as a lot of attacks that otherwise would have missed will work to dissolve the images.
So if we assume that there is no large gap between the AC of the images and the true form, then the spell essentially turns at most 3 hits into misses. So representing the bonus of the spell with a change in effective HP is much more accurate and easier to calculate, just add 3x the usual damage of a PC attack (before applying the multiplier for resistances).
Now, this will swing a bit depending on the power level of the monster. Low HP monsters might not get the chance to take full advantage of this, so you might want to lower the 3x multiplier. The damage from a PC hit will also change with the level at which the monster is encountered.
So for a CR<1 monster I would add about 2x (d8+3) = 15 effective HP. For a tougher ~CR5-10 creature I would add 3x (d8+d6+4) =36 effective HP.
Note: if the monster has to use an Action to cast the spell, do not forget to adjust the damage calculations too, as it will not be using that Action to attack.