So I finally finished my Monster CR calculation spreadsheet and plugged the Spirit Naga in. A few interesting things came out of it. The only way I was able to get to CR 8 was to:
*Use lightning bolt for 3 rounds (1 at 5th level, 2 at 4th level) and average hitting two targets each round
*Use the immunity to poison to double the effective hit points even though there is only 1 immunity (poison) instead of several.
Using lightning bolt at the maximum levels possible was not what I first looked at to figure out the most effective DPR, but it turned out to be the right answer in the end. I would not have expected to need to use the damage immunity to double the effective hp since the guidelines say to only use the effective hp multipliers if there are several immunities or resistances. However, it was the only way I could get to the Spirit Naga's CR of 8.
So, setting the spreadsheet up with the Spirit Naga's stats so that I came up with a CR of 8, I adjusted the actual HP up by 150. If the doubling of effective hp had not been there, it would have only resulted in a modest CR bump of a few levels. I was looking for a CR of 17. However, because of the doubling of effective hp, the Spirit Naga's effective HP was 450. When everything averaged out, the CR was 16. So this was interesting. Just bumping HP by how many CR levels you want to change may work, but it would require more checks of creatures to see if it worked regularly or if this was dumb luck.
*Use lightning bolt for 3 rounds (1 at 5th level, 2 at 4th level) and average hitting two targets each round
*Use the immunity to poison to double the effective hit points even though there is only 1 immunity (poison) instead of several.
Using lightning bolt at the maximum levels possible was not what I first looked at to figure out the most effective DPR, but it turned out to be the right answer in the end. I would not have expected to need to use the damage immunity to double the effective hp since the guidelines say to only use the effective hp multipliers if there are several immunities or resistances. However, it was the only way I could get to the Spirit Naga's CR of 8.
So, setting the spreadsheet up with the Spirit Naga's stats so that I came up with a CR of 8, I adjusted the actual HP up by 150. If the doubling of effective hp had not been there, it would have only resulted in a modest CR bump of a few levels. I was looking for a CR of 17. However, because of the doubling of effective hp, the Spirit Naga's effective HP was 450. When everything averaged out, the CR was 16. So this was interesting. Just bumping HP by how many CR levels you want to change may work, but it would require more checks of creatures to see if it worked regularly or if this was dumb luck.