An instant, over-powered attack by a boss type opponent is a bug. If you are playing the Developer's Cut release of the game, the easiest solution would be to load the save in the Developer Mode version and enable god mode until you get past that fight.
If you are playing DKS, lowering the difficulty level may help (it will reduce the damage done, hopefully enough it is no longer a one hit kill).
Perhaps drink an armour and/or healing potion just before this fight. If you happen to have any jewellery with a summon bonus, having an extra possible target could help. Even if you don't have time to move or pause the game (and use Hide in Shadows, or something), Ba'al may decide to target a summon or your creature, giving you time to attack (or if you can stay mostly behind your creature, it may block a spell cast at you).
One person who ran into this problem used CheatEngine to fix their health until they got past Ba'al (though someone mentioned that Steam may have a problem with the use of CheatEngine, even with a single player game).
At least one person just loaded a save before the opponent they were having trouble with, went and did other stuff, and when they got back to trying it again the boss didn't have an instant / overpowered attack.Where's the trick to Ba'al?[Not Sure if Bug][Spoilers]Killed Instantly
If you email your save to firstname.lastname@example.org, with a description of the problem, they can fix it for you.
In XP the saves are in the folder
C:\Documents and Settings\ %username% \Local Settings\Application Data\Divinity 2\Savegames
or in Vista / Win 7
C:\Users\ %username% \AppData\Local\Divinity 2\Savegames
You may need to set Windows Explorer to show hidden files and folders (XP, Vista
, Win 7