Last season my football team lost against a team 1-0. The next time we faced them we did not revise our tactics, simply went in knowing we had to win. Our tactics work most of the time, and us losing wasn't due to us being terrible at what we do, or going in without a plan. We faced them, knowing we had to win, and knowing we had to be first to the ball every time, and simply want it more. We beat them 4-0.
Buffy intentionally deciding not to bring weapons or use anyone's help makes it more of a case like that of a football team losing a game after two players got sent off and deciding for some reason to play the next game with nine men, just to prove they can win that way. Even if they miraculously win, this wouldn't make their plan any less risky and stupid.