Heracross will either have it when you catch it or you can travel to Pastoria City and go to the east side of town(the house next to the water) to the move relearner. You will need a heart scale. Talk to the guy in the house and he will allow you to get heracross night slash.
only heracross' good moves are listed here: * night slash * megahorn * close combat * brick break * horn drill
I think night slash because more Pokemon can learn slash but few can learn night slash if it weren't a TM
Sceptile, to my knowledge, is incapable of learning Night Slash.
Absol learns the move Night Slash at level 41. Absol also learns the Psychic type version of Night Slash called Psycho Cut at level 47.
See related links section.
Gallade can learn night slash, but not by level up. To teach Gallade night slash the player most go to Pastoria city and go to the south part of the city then go into the house at the top right of the southern part of the city. There is a guy who can teach Gallade night slash for a heart scale and all other moves it learns before that. I hope that helped!
95
Heracross learns megahorn at level 55
heracross learns close combat at lv.37
First, do not evolve your sneasel, second, find a heart scale, third go to the move relearner's house. Give him the heart scale and select your sneasel to relearn night slash
Sources say... Night Slash...
heracross learns close combat at a level of 37.