Finally did it.

Patience and preparation helped me to accomplish my holy order :)

1.) Raised power level to >670 which gives roughly 35% more damage and less damage taken.

2.) Cleared the whole level before talking to Flavius. Important note: get the additional soldiers after clearing the whole area and before talking to flavius, they die to quickly as well but provide good fire support later on.

3.) I placed the mines to the right (looking away from Flavius) and the turrets on the left side.

During the fight stay around in the middle and try to draw damage to your character. Flavius will stay in the back. Once he gets taunted by any mob, better kill everything very fast otherwise he'll stay in the fight and dies. That was my primary issue.

Worked.

Anyway this is a tough mission :)

Thanks for your comments. That's what i currently do, doing some mission, gain level and powerlevel, try different weapons. 

Issue is less my actual gearlevel, it is more or less that Flavius Draken is dying very quickly. 

If he would stay in the back i wouldn't mind either but he has too few hitpoints to deal with the mess :)