Most 1v1 matches are decided within a few turns, which means brute force is often the way to go. Moves that aren't good in other formats become quite solid in this format, like Hyper Beam, Frenzy Plant, etc. With the nukes that are Frenzy Plant, Leaf Storm, Giga Impact, and Hyper Beam, Sceptile is quite able to OHKO an opponent, and win the match. Its Speed tier will allow it to outspeed quite a few Pokemon, and if you put, say, a Sitrus Berry on it, it can also get an Unburden boost. It gets a wide range of moves, including things to cover every one of its weaknesses.
In Generation VII, it gets a Mega that grants it immunity to Electric moves, preventing it from being paralyzed by most moves. It was also relatively faster in Generation VII, as quite a few Pokemon are faster than Sceptile in Gen VIII. It can nuke an opponent even harder with a Z-Crystal, or stall them out with Leech Seed and Grass Whistle. Using Substitute a few times will allow you to obtain an Overgrow boost, making it even more dangerous.
I'm no expert, but overall, I think Sceptile is pretty solid in 1v1.
Hope I helped!
