While building some non canonical adventures for my players I came up with the idea of statting a few dragons I want to use as "last bosses" in my home games. While doing this isn't too difficult I worry about the CR since I'm unsure about applying it correctly.
I have 3 dragons who are mythic and also have character levels. They are:
-A Gold Great Wyrm, Alchemist (vivisectionist) 20, Archmage 7.
-A Red Great Wyrm, Sorceror 15 (Arcane Bloodline), Archmage 10
-A Blue Great Wyrm, Mesmerist 20 (Enigma), Trickster 8
What would the suggested CR for these NPC villains be?
We are well outside the boundaries of what CR is intended to measure here, so I'm just going to go with a straight by-the-books answer according to the guidelines for adding monster levels
A Gold Great Wyrm, Alchemist (vivisectionist) 20, Archmage 7.
CR 43 MR 7
CR 23 for the Gold Great Wyrm, +20 for 20 Alchemist levels (which should add in full).
A Red Great Wyrm, Sorceror 15 (Arcane Bloodline), Archmage 10
CR 33 MR 10
22 for the dragon, +1 for the first level of Sorcerer, +10 for the remaining 19 levels of Sorcerer. This is because only the first level of Sorcerer advances his spellcasting, after which his Sorcerer spellcasting is maxed (Red Great Wyrms already cast as a 19th level Sorcerer, so one more level maxes it out) so the remaining levels have half value.
A Blue Great Wyrm, Mesmerist 20 (Enigma), Trickster 8
CR 31 MR 8
21 for the great blue wyrm, +10 for the 20 levels of Mesmerist (which is not a key class, in my view)