DC51 will save thats just good enough but not much more , since they have +45 will save AND 45 sr , 20 sr good by the end . They dont have to roll good to make it... Do your icy spells bypass spell resists ? Else its very likely to not work.I am also pretty sure everything is stat drain immune, since exsanguinate was only doing damage.
Most of the higher end enemies aren't immune to everything though, just to "most" things. But they all have a few weaknesses that you can exploit, and Expanded Arsenal makes dipping into other spell schools super easy. Undead/Serpentine bloodline allow you to charm/compel enemies that have natural immunities against it, too. It's not an end-all-be all solution, ofc, but it works against most enemies. Also, as I said, 51 is just without buffs or demonic rage. 7th level Enchantment spells are DC 56 for me right now.
Spell resist is a non issue, and keep in mind that persistent metamagic or Socothbenoth Aspect allow you to cast every spell with advantage.
It's not the toughest enemy, but one of the many people are complaining about here. Keep in mind that I'm only level 18, and am still using Chapter 3 items, like the Battle Mage Quarterstaff. I'm sure there's better end-game items that will let me further boost DC.