Okay, then I had it slightly misconstrued: You can stun Raze while his HP are sufficiently high enough not to trigger the healing cutscene yet. Then, while he is stunned, you pull both levers to switch off the engine. Now you have him at the lowest possible HP with the machine switched off - and then you have to be fast enough pummeling him down to 0.
The most difficult part IIRC is actually moving across the room fast enough to switch off both levers at once. Sometimes you can use the living armours to have a target for charge attack, and use that to get to the other lever quickly. Otherwise, you'll have to jump over their heads, which costs you valuable time.
About Sassan: Well, she hit me with that wizard skill that does multiple direct hits ("magisches Trommelfeuer", don't know what it's called in English), so technically it was not "one-hitting", but it was a single skill use - and since that skill hits you directly, you cannot evade it like you can with a fireball or magic missile.
