To recap my understanding...
- The point of Adjusted XP is provide a metric with which to judge the difficulty of a combat, to give your party well-balanced encounters and well-balanced adventuring days overall.
- The encounters for the day "should" sum to the Daily XP Threshold; Notably, multiple separate encounters don't increase the Adjusted XP for each Encounter - that's accounted for in the Daily Threshold.
- For a single monster encounter "Adjusted XP" = "Monster XP".
- Obviously (due to action economy) fighting multiple monsters simultaneously is disproportionately hard: Fighting 2 Werewolves is more than twice as hard as fighting 1 Werewolf.
- The Encounter Multipliers table accounts for this:
- So 2 Werewolves are 3 times as hard as 1 Werewolf.
End of Recap
This concept works great when your encounter is "n copies of the same monster".
Question: How do I calculate the Adjusted XP for a mixed monster encounter?
It's trivially obvious that just applying a single Multiplier to the whole lot doesn't work.
Consider an extreme case of fighting a Adult Red Dragon plus 2 Giant Rats. By RAW that encounter is now 2 x (18,000 + 25 + 25) = 36,100 Adjusted XP! But it's completely obvious that this encounter isn't even slightly comparable to "fight an Adult Red Dragon ... and then after you've killed it, its mate appears", which would also be a total of 36,000 Adjusted XP.
On the other hand, in a less lop-sided encounter, say ... a Tree Blight flanked by a handful of Needle and Twig Blights ... there is a material impact to having to deal with them all at once, so you couldn't just use "apply the multiplier to groups of the same CR, and then add the results together.
So ... are the any published, or common, ways of handling Adjusted XP for heterogenous monster groups?