What character builds are you using?
Have you finished everything else in that act? If not, reload a save before giving the ingredients to the alchemist, and go finish other things to level, or visit the BF repeatedly to look for better equipment, etc.
After he teleports you to another location, if you approach the area he is in slowly, you should be able to draw him to you without involving the two water demons there.
Some people have found the Hammer skill quite effective against the shaman, if you know or can learn it (the hermit in the spider area can teach it up to level 5, and Elkar in the BF can teach up to level 2).
The act 1 summoning doll can equip a crossbow, and kept out of direct combat can provide good support in tough fights. Other summoning dolls could at least provide a distraction, though even upgraded are not particularly tough.