Using the Sorceress, make sure you are atleast level 20 when you begin Extreme Mode with any character class, it'll make it a tad bit easier, also, make sure you have Ball of Lightning spell upgraded, best spell to use on enemies I think.
Karne: This guy isn't all that tough really, lots of hit points, and some painful attacks. As you know, it's easier just to stay away from him, using Mordankiennens? Sword, will do some damage, but, your going to have to use a spell that'll hurt him.
Ball of Lightning doesn't take much energy, but it only does half damage on bosses, but, it'll keep you from getting injured on this guy.
Beholder: My suggestion, is just to stay close to him, his spells hurt, and so does his bite, one chomp, good bye, so just stay close and beat the crap out of him, blocking when he goes to strike.
Illuvia, errr, whatever her name is: Now, I really didn't discover a weakness, she's tough, and her teleportation is annoying. Same as before, try to use your ball lightning to damage her, it'll take a while, but you'll get through, and use your Mordankiennen's Sword to injure her as well. You can get close if you want, just don't stay there too long, also, let her summon some skeletons, your lightning will take them out quickly, they add extra experience, and if your lucky, healing or rejuvanation potions.
Ice Dragon: This guy is a wimp. Just use your lightning and sword spells, stay back, and out of range of his breath spells, they'll kill you before you know it, but he's not all that tough, so no problems.
Sess'sth: Hahaha, this one is a pain. There is no time to cast a lightning spell on him, for he is too fast, and you can't block his attacks, he hurts you anyways. You CAN cast your Sword spell, but, you have to time it right, or you'll be dead, just run around him, hitting him here and there, and cast that spell when you've got time, just keep in mind, he's got I'm guessing well over 3000 HP, so careful.
Iron Giant: One suggestion, lightning spell, and distance, okay two. He's no problem.
Last Boss: Now, she is immune to lightning, and you don't want to get close to her, she'll chop you down in a heart beat. In the early stages when she just throws her sword, it's alright to stay close to her, and using your Mordankiennen's Sword spell. The spell I found most effective, was the Melf's Acid Arrow fully upgraded. It'll do around 20 damage on impact, and 5 every second, so it'll take her health down quickest. Second stage, swords from the sky, just move around and use your Sword and Acid arrow spells. Now, her one attack is that she fires lightning from herself, and damages you, I think it regenerates her health, but I'm not so sure about that one, never payed any attention to that.Just remember to stay away from her so that she isn't running around. Try to keep her in one spot. When her health bar is about this full.....|--------| she'll start using that spell alot, so just stay out of range of it, and use your acid spell, you won't be able to see her where your standing, so your going to have to follow the lightning back to the caster and make sure your lined up. That's my suggestions, but then again, I have a level 30, so yeah, wasn't too hard, mwuahahahahahhaha. Good huntin'!