Thank you for your replies!
I wont use Fatepoints for opponents, thats something that makes the characters even larger than (they already are) life.
And we play with reduced Fatepoints. They had two for the entire campaign...
The idea with the reducing of damage caps works fine. But then we have the open-ended-roll.... doesnt really help.
I did let the situation go on and the fighter is called "Dragonslayer". Of course. Everybody is impressed, stories are told and now he's most wanted... even new enemies...
As the GM I wouldnt decide against good-players-luck, still the disappointment is there. I'm not always the groups' best friend, in bossfights I'm the boss to be fought...