I rarely am able to stand toe-to-toe (or tentacle) with the soul eater. I have been able to use "hit and run" tactics ... go into battle, hit him (or her, it, whatever) for a hundred or so damage, then run. I usually do a lot of running anyway, since my party is always turned against itself, and by running far enough, they won't act.Then, when the souleater breaks off chasing you, heal fast (don't rest, cause it heals while you do), and run back in for more.
The next group (greater demon and djiin of clouds) I don't think I have ever beat ... their death cloud is usually to much. Funny, it was probably on my 4th or 5th time through the game before I got the implied meaning of their name 
I can usually take out the demon, w/o too much trouble. I always sneek as far as I can (on the right hand side) and make my way (either with or sometimes w/o engaging in combat), into the temple. Then, I will peek out the door and fire off any cloud spells I can and/or area effect spells. This will usually engage the demon and sometimes one or more of the Djiin.
The Demon will usually travel more and come right up to the doorway, where you have an advantage. As you continue to fight him, and the Djiins can't see you, they usually bow out of combat. If you could get them to attack individually, you could use this tactic to pick them off too, but I have yet to have that happen ... once one of them can see you, they all target you with clouds.
Or, you could just run 
Tools for Wizardry(r) 7