請問這句句子的文法有沒有問題?

'Pray to my hammer, little guy!' the fatty grinned, raising his hammer,and, I was sure, would chop me into two pieces in no time although I didn't see what he was doing personally since that wasn't necessary－I could imagine that!

Yellow portion:
Assuming that "I was sure" was an interjection, the sentence becomes: "the fatty grinned, raising his hammer, and would chop..." The "would chop" part suggested that a hypothetical situation, which did not flow well in the context. You can fix the problem by using "getting ready to chop..."

Green portion:
I felt that you had squeezed so many independent and incompatible events into one sentence, making it very difficult to be understood. You said that you were sure that you would be chopped into half, but then two seconds later you said that you did not see what fatty was doing.

Pink portion:
I cannot tell what "that" refers to. My guess is fatty's grinning and raising the hammer, but I don't see the relevance with "not necessary."

Blue portion:
I cannot understand what "that" refers to. What could you have imagined?

This sentence has no grammar problem :
Although it is not a good way of writing a sentence with so many information and actions inside, plus the feeling of the writer in just one sentence. Nevertheless the writer did not make any mistake in the rational nor reasoning of all the actions and adjectives.

"Pray to my hammer, little guy!", the fatty grinned while he was raising his hammer. I was sure it would chop me into two pieces in no time without seeing what was he doing. It wasn't necessary because I could imagine that!

I am sorry that I can't be fully understand what you wrote. From my understanding, it should be written as above.

Good luck :)