If you agree to kill Aurelius, Svadilfari shouldn't turn hostile until after you return to complete that quest...
In any case, lowering the difficulty level may help, if you are not already playing on casual (it will reduce the damage done, hopefully enough it is no longer a one hit kill). If you leave this until later, a higher level and better equipment may help you survive the initial attack, though (without spending time in Keara's headquarters spawning opponents) I doubt you would be able to get more than another 4 levels before the only thing left to do was entering the Hall of Echoes.
Do you have your creature active? Try getting as close as you can without triggering the cutscene conversation, summon the creature (and use a summon spell, if you know any, or have any jewellery with such a bonus), perhaps drink an armour and/or healing potion, then start the conversation. Even if you don't have time to move or pause the game (and use Hide in Shadows, or something), Svadilfari may decide to target a summon rather than your character, giving you time to attack (if you can stay mostly behind your creature, it may block a spell cast at you).
For the retail version, do you have the
patch installed? Any recently downloaded digital version should already be updated.
This problem should be prevented in an updated game, but if it happens in an unpatched game, installing the patch will not fix it if the issue has already occurred.
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).
Where's the trick to Ba'al?[Not Sure if Bug][Spoilers]Killed InstantlyIf you email your save to support@larian.com, with a description of the problem, they can fix it for you (though not until next week).
With the PC version, 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).