Pretty sure this is covered in the rules, but I don't have them with me to confirm or give page number.
From memory, if the base score was 7 you will re-roll any 6s you get needing 4,5 or 6 to get a hit. Same procedure if you needed an 8, but the second roll (of any 6s) would have to be a 5 or 6 etc. Try as hard as you like, you won't manage any kills if the original to hit number was a 10