It’s a lot tougher battle than most of you would think.
Yes, the Bat was able to beat Superman. But then Superman has a major weakness that Batman, can use to his advantage - called Krytonite. Buffy doesn’t have such a weakness. Yes, Batman’s smarter and has lots really cool toys. But, Buffy has the advantage of superhuman strength, and agility. Yes, the Bat has some major foes(all of whom are basically human). Buffy faces powerful superhuman foes on regular basis.
Buffy may have a few surprises for the Bat, in the fact, that he likely would think such a small girl could be so strong and fast.
And as for Buffy, Angel, Faith & Willow vs. Superman & Batman. Here’s how I see it. Willow immobilizes Superman(remember that magic is his other weakness), then Buffy, Faith, & Angel take out the Bat.