Here are some definitions from
UsingEnglish.com:
Barrel of Laughs
If someone's a barrel of laughs, they are always joking and you find them funny.
Over a Barrel
If someone has you over a barrel, they have you in a position where you have no choice but to accept what they want.
Since the idiom is said to refer to a person rather than an object or a situation, I would change
It was not a barrel of laughs to
He was not a barrel of laughs, for example.
I suppose the second one is correct when put in context.