We had the above comment posted on our Hissho (Must Win) hachimaki listing on Amazon the other day. Personally, I think "must win" means something very similar to "certain victory" so I am not sure why the commenter thinks our translation is not acceptable.
"Certain Victory" is a very literal translation of the characters 必(hitsu) and 勝(sho). In the context of someone wearing it on a hachimaki, however, the word hissho is describing the wearer’s determination to succeed or win and hence our translation to "must win". "I must win" would be another way to put it.
Many words cannot be translated into another language in an easy way (see this article). Perhaps Hissho could be regarded as one of these!