Overpowered, nearly instant attacks are a bug some people have run into with particular boss opponents.

Is there a god mode in the Developer Mode version of the game? I played around transforming my character, but not much else (there are separate shortcuts for the normal version of DKS and the version with the Developer Mode). Really, though, invulnerability should be an obvious feature for any self respecting dev console.
With DKS, one person who ran into this problem used CheatEngine to fix their health until they got past Ba'al (in the mine, in that case).

Someone in the Larian forum playing DKS reported that if they paused the game immediately after the conversation ended, went into their inventory and used the shield in the quest item section, Laiken and Razakel's damage dropped back to normal. I don't know if anyone repeated that, but it is worth a shot.

Lowering the difficulty level may help, if you are not already playing on casual (that will lower the damage done, hopefully enough it is not an instant kill), as could drinking healing / armour / stat potions just before the fight.

When the conversation ends, Hide in Shadows may help, if you know it (possibly from a bonus on a piece of jewelery), or have a skill point to learn it. Your creature and possibly a summon might take care of them. A Stun Arrow, or possibly a Rush Attack, or something, may let you finish either off before they can hit you, though if they both have over-powered attacks it may be tricky taking out both of them without getting hit.

Even if you are just behind your creature or summon, Laiken and Razakel may decide to target them first rather than your character, or they could block an attack meant for you.

Where's the trick to Ba'al?

[Not Sure if Bug][Spoilers]Killed Instantly

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If nothing else helps, you can email your save to support@larian.com, 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).