Need help figuring out the CR on a custom bad guy

[Gwarok]

I'm making the BBEG in a dungeon crawl essentially a Storm Giant with 11 levels of Sorcerer and troll regen, with immunity to fire. It's a Frost Troll type guy, but Fey instead of giant. Anyone know what or wanna guess what the effective CR on that would be?

 

[The Crimson Binome]

Off the top of my head, I would guess 17, but only if those Sorcerer levels give them an addition 11d12 hit points. Otherwise, the regen and counterspells just up its CR by about +1.

Except for Counterspell and HP, eleven levels of sorcerer don't add much, considering that storm giants already have strong attacks and lightning strike. Counterspell, specifically, would let it maintain regeneration in the face of Chill Touch or Acid Arrow.

 

[aco175]

The storm giant base is 13 and I find that casting spells adds 1 for every few levels of spells you can cast. You cast 6th level spells at 11th level so that adds 2 maybe 3 with the regeneration. I would give it a 15 if you have a well designed party or 16 is more standard.