Clearly CDP is unable to balance gameplay of his games as I beated the game at Death March but keep dying at Deatlaff fight even at lowest difficulty.
Is there a god mode I can enable?

add the line DBGConsoleOn=true to the general.ini file in The Witcher 3 Wild Hunt\bin\config\base.
https://www.gog.com/upload/forum/2016/08/8406f001776c21c0749c9053e3436308b24a4167.jpg
Then in game hit the ~ key and type god
Doing it again toggles it back off.

Thanks, I managed to beat him at lowest difficulty without the need of godmode (but had to read tactic)
Anyway Blood and Wine dlc is really a piece of crap gameplay/balance wise, going from too simple to too difficult fights is never a good thing and also I like trying to understand how to fight a boss but not if I keep dying without being able to try some tactics first.
I think that Deatlaff is one of the dumbest and badly designed boss fight I have ever seen.

He's a high vampire, a creature most witchers don't like to take a contract on. He's suppose to be extremely though.
That sad, doc Hubert in the base game wasn't that hard.