Any tips on dealing with theses Blood Knight with the Sorcerer?
I'm getting slaughtered by them, only thing that seems to hurt them is fire and they've got a a good resistance against it, it's taking so many fireballs just to kill one. I guess it's just a case of buying 100s of mama potions and battle on?

Correct. Blood Knights are Immune to Magic and Lightning, and Resistant (75%) to Fire. Fireball is your best choice, though careful use of a high level Fire Wall might let you shape their movement. You may also benefit from careful use of Teleport to allow you to open up the range. Cast fireballs on them as they approach, then teleport backward when they are near melee range. Repeat as needed. Finally, if you have sufficient mana, Stone Curse can be very helpful, since it will stop their movement, making them an easier target, preventing them from attacking you, and cause them to impede the path of their allies.

Stone curse and golem. Even on normal difficulty they have high resistances/immunities. Your sorc will also need those spells if you plan on playing higher difficulties.

i have a fairly look level fire wall, but the fireball is quite low as i haven't found many books, not even for sale.
don't think i have stone curse or Golem . I tried flame wave as it can hit several at once but it never touched them.

So far i've been casting several rows of fire walls, and then just blasting them with the fireball i do have, firebolt is high level but that doesn't touch them.

Good job i've been hording coin, seems i'm going to need many mana potions to finish it.

Something you should know. Adria restocks her store inventory every time you enter town. So if you need further fireball level, you can just keep reentering until she has a fireball book for sale. You just said you have a good supply of cash. The only thing is that books are somewhat less likely to appear for sale than other items, so to find a particular book requires a lot of patience. Last time I played a sorcerer, I certainly tried mine (but it was worth it to be able to complete the game without too much other trouble).