Can you kill Sans?
Correspondingly, can you beat Sans?
Sans has 24 attacks, but a couple of them repeat often. Sans starts off the fight with his "strongest move," which can catch many new players off guard. But it's always going to be the same move so once you learn how to dodge it, you can dodge it as many times as you want.
Also asked, how many attacks does it take to kill Sans?
There are only 13 lines of dialogue after Sans finishes the first attack, the only way to advance is to fight, which means you must attack Sans 14 times in order to get to the half-way point.
It is possible depending on what you mean by “first-try”. First-try with experience on fighting Sans: It's possible. If you have experience in fighting Sans, you'll know what attacks will strike, when the attacks will strikes, and how. First-try without ever seeing the fight: It's next to impossible.